feat: Add support for styling Journey Diagram title (color, font-family, and font-size)

[Shahir-47]

📑 Summary

This PR introduces additional customization options for the title in Journey Diagrams. Users can now define titleColor, titleFontFamily, and titleFontSize in the themeVariables configuration to style the title's color, font family, and font size. If these properties are not defined, they default to textColor, "trebuchet ms, verdana, arial, sans-serif", and 16px, respectively. The feature includes corresponding updates in the rendering logic and documentation.

Resolves #3508

📏 Design Decisions

1. New Theme Variables:

   • titleColor: Sets the color of the Journey Diagram title. Defaults to textColor.
   • titleFontFamily: Sets the font family for the title. Defaults to "trebuchet ms, verdana, arial, sans-serif".
   • titleFontSize: Sets the font size for the title. Defaults to 16px.

2. Renderer Updates:

   • The Journey Diagram renderer now fetches these new properties using getConfig() and applies them during rendering.
   • Relevant code changes:

     const titleColor = getConfig().themeVariables.titleColor;
     const titleFontSize = getConfig().themeVariables.titleFontSize;
     const titleFontFamily = getConfig().themeVariables.titleFontFamily;
     
     if (title) {
       diagram
         .append('text')
         .text(title)
         .attr('x', LEFT_MARGIN)
         .attr('font-size', titleFontSize)
         .attr('font-weight', 'bold')
         .attr('y', 25)
         .attr('fill', titleColor)
         .attr('font-family', titleFontFamily);
     }

3. Default Values:

   • In packages/mermaid/src/themes/theme-default.js:

     this.titleColor = this.titleColor || this.textColor;
     this.titleFontFamily = this.titleFontFamily || '"trebuchet ms", verdana, arial, sans-serif';
     this.titleFontSize = this.titleFontSize || '16px';

4. Documentation Updates:

   • Updated the Theme Variables section in packages/mermaid/src/docs/config/theming.md to include titleColor, titleFontFamily, and titleFontSize.

---

📋 Tasks

Make sure you

• [ x ] 📖 have read the contribution guidelines
• [ x ] 💻 have added necessary unit/e2e tests.
• [ x ] 📓 have added documentation. Make sure MERMAID_RELEASE_VERSION is used for all new features.
• [ x ] 🦋 If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpm changeset and following the prompts. Changesets that add features should be minor and those that fix bugs should be patch. Please prefix changeset messages with feat:, fix:, or chore:.

Updated Documentation

File Updated: packages/mermaid/src/docs/config/theming.md

Added the following rows to the Theme Variables table:

| Variable             | Default value                      | Description                                                                                                                      |
| -------------------- | ---------------------------------- | -------------------------------------------------------------------------------------------------------------------------------- |
| titleColor           | calculated from textColor          | Color to be used for the title text in Journey Diagrams.                                                                         |
| titleFontFamily      | trebuchet ms, verdana, arial       | Font family to be used for the title text in Journey Diagrams.                                                                   |
| titleFontSize        | 16px                               | Font size in pixels to be used for the title text in Journey Diagrams.                                                           |

---

Example Configuration

Users can customize the Journey Diagram title using the following configuration:

---
config:
  themeVariables:
    titleColor: "#ff0"
    titleFontFamily: "Arial, sans-serif"
    titleFontSize: "20px"
---

journey
    title User Journey Example
    section Onboarding
        Sign Up: 5: John, Sam
        Complete Profile: 4: John
    section Engagement
        Browse Features: 3: John
        Use Core Functionality: 4: John
    section Retention
        Revisit Application: 5: John
        Invite Friends: 3: John

With this configuration, the title "User Journey Example" will have a yellow color, Arial font, and 20px font size.

---

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[autofix-ci]

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[sidharthv96]

Please add visual tests that verify the parameters work.

[sidharthv96]

Do not call getConfig() every time, call it once, and then use the value in other places.

[pranavm2109]

@sidharthv96 Understood; we (@Shahir-47 and I) will make sure to update this by calling getConfig() once and then using the value in the other places. As for visual/unit testing, could you please direct us to resources that clarify what kinds of visual/unit tests are expected?