Update YAML editor to provide default content

If the user switches to YAML mode in the Create PipelineRun page we
should not present them with an empty editor. At the very least we
should pre-populate it with the basic outline of a PipelineRun, i.e.
`apiVersion`, `kind`, `metadata` with default name + namespace
placeholder, `spec`.

If the user has already updated the form before switching to YAML
mode, preserve their selected inputs and display the corresponding
YAML so they can continue editing as needed.

Remove unused code from the YAML editor related to loading of the
Pipeline resource.

package.json

@@ -36,6 +36,7 @@
     "carbon-icons": "^7.0.7",
     "core-js": "^3.26.1",
     "git-url-parse": "^13.1.0",
+    "js-yaml": "^4.1.0",
     "lodash.clonedeep": "^4.5.0",
     "lodash.keyby": "^4.6.0",
     "prop-types": "^15.7.2",

packages/e2e/cypress/e2e/run/create-pipelinerun.cy.js

@@ -97,6 +97,7 @@ spec:
     cy.contains('button', 'YAML Mode').click();
     cy.url().should('include', 'mode=yaml');
 
+    cy.get('.cm-content').clear();
     cy.get('.cm-content').type(preserveIndentation(pipelineRun));
 
     cy.contains('button', 'Create').click();
@@ -131,6 +132,7 @@ spec:
     `;
     cy.visit(`/#/pipelineruns/create?mode=yaml`);
 
+    cy.get('.cm-content').clear();
     cy.get('.cm-content').type(preserveIndentation(pipelineRun));
 
     cy.contains('button', 'Create').click();