Implement BATS workflow

[Stealthii]

This PR implements the bats test runner as a workflow, providing a stub test to ensure passing until future PRs implement their own.

The purpose of bats for this repo is three-fold functionality testing:

• multiple Bash versions (4.2, 4.4, 5.0, 5.2)
• multiple OS flavors (macOS, Linux)
• Alternate versions of bc, awk, git, date, etc. (where necessary)

Tests are completely optional, and realistically should only exist for core components where functionality differs between OS flavors (such as the music widget), and non-portable syntax (use of implementation specific features in bc, awk, date, etc.)

Discussion in comments.

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Minor changes

• Update known dependencies for tokyo-night-tmux
• Provide dependency installation examples for macOS, Alpine, Arch, and Ubuntu

[Stealthii]

Main goals I'm planning to achieve:

• Functionality for Bash 4.2+ (5.0 got stricter with variable expansion, as an example)
• Eliminate dependency on GNU coreutils, bc, awk, sed, echo, etc. (writing portable code)
• Highlight PRs that break functionality or add dependencies
• Highlight performance issues

I plan on raising separate PRs to facilitate this, which is why I'm raising this as a base for those PRs for now.

[Stealthii]

The rationale on targeting Bash 4.2 as a minimum:

• It supports the \u and \U Unicode escape sequences within echo/printf (which many theme components utilise)
• Provided in CentOS 7 (EOL 2024-06), Ubuntu 16.04 (EOL 2021-04)

The only supported OS shipping with an older Bash is macOS (3.2) which is already too old to support other features in this theme (like associative arrays) - and Homebrew has become the de-facto package manager on this platform - even Github Actions uses it in the base runner instances. With the update to the README reflecting dependency examples, this should be less of a problem moving forward.