ESLint: enforce trailing commas and html indentation

[MetRonnie]

Enough is enough 😄

See also #1536

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Tests, changelog and docs not needed
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[oliver-sanders]

Enforcing trailing commas is perhaps a little funky, but whatevs.

Perhaps wait until we've flushed out the PR backlog and retry?

[MetRonnie]

Enforcing trailing commas is perhaps a little funky, but whatevs.

We are already enforcing dangling commas in JavaScript file, this extends it to Vue SFCs. (As ESLint is a formatter, editors can be set up to automatically do this on save, or by eslint --fix)

[oliver-sanders]

We are already enforcing dangling commas in JavaScript file

Wow, hadn't even noticed?! But I may just be doing this anyway.

[MetRonnie]

Ah no I was wrong, before we were merely allowing dangling commas when on multiline arrays and objects (but if a trailing comma was left on a non-multiline array/object, the formatter would strip it). Now I've set it to enforce for multiline arrays/objects so that if a trailing comma is missing from a multiline array/object the formatter will add it (non-multiline dangling commas will still be stripped. This should make life easier as you can add entries to multiline arrays/objects without having to fiddle with missing dangling commas on the line above.

[MetRonnie]

Perhaps wait until we've flushed out the PR backlog and retry?

So wait until 5 years time? 😅

Just force-pushed again