🏗 Add amp validate-html-fixtures to AMP's PR check workflow

[rsimha]

PR Highlights:

• Introduces a new PR check called amp validate-html-fixtures to validate all AMP test fixtures
• Automatically determines the HTML format of the files being checked (AMP, AMP4ADS, or AMP4EMAIL)
• Checks HTML fixtures for validation errors using amphtml-validator --html_format <format> <files>
• Adds Targets.HTML_FIXTURES to build-targets.js to trigger the check during CI only when necessary
• Adds amp validate-html-fixtures --local_changes to amp pr-check for local development (Validation tests for examples and test fixtures #25149 (comment))
• Fixes a few files to address parse errors and to serve as an example for future PRs (Validation tests for examples and test fixtures #25149 (comment))
• Temporarily allow-lists files that need fixes in build-system/test-configs/config.js (Validation tests for examples and test fixtures #25149 (comment))

Note: Here is the full list of validation errors in our HTML fixtures. Fixing all of them will require a gradual team-wide effort.

Screenshots:

Failing check (single file):

Failing check (local changes):

Passing check:

Partial fix for #25149

jquery is no longer being introduced as a devDependency. Nice catch!