bug 1354243: add macro linter (for JSON "macros")

[escattone]

• expand the make lint-macros command to include the linting of JSON data files within the macros directory (using the newly added jsonlint-cli)
• add make lint-macros to the "Test" stage of the Jenkins pipeline
• fix JSON syntax error in macros/L10n-HTML.json so make lint-macros runs clean

[escattone]

I wanted to also add a CSS linter (for the one CSS file within the macros directory - there are actually two files that have the .css extension, but macros/L10n-CSS.css is actually a JSON file and should probably use the .json extension instead?). I went down the path of trying stylelint, but realized I was a fish-out-of-water when it came to setting up a strictly-syntax-error-only configuration. If we think it's worthwhile, I can work later with @stephaniehobson to set that up.

[stephaniehobson]

I did a quick look for a stylelint configuration we could borrow just to check for syntax errors but didn't turn anything up. I don't know how long it would take to put together a simple syntax check ourselves.

On the other hand, it could be argued that we should not have any CSS in this repo at all. Not even inline styles.

[jwhitlock]

I agree:

• no CSS linting is required
• L10n-CSS.css should be renamed L10n-CSS.json
• CustomSampleCSS.css should be deleted. According to the macros dashboard, it is unused on MDN.

[escattone]

Thanks for the feedback @stephaniehobson and @jwhitlock !

• renamed L10n-CSS.css to L10n-CSS.json
• deleted CustomSampleCSS.css

[jwhitlock]

Tested locally, with good JSON in PR and by adding some bad JSON and getting an error. Good fixes to JSON, and abolishing CSS from the repo. ❤️ this PR.