languages: Prefer arrow callbacks in ES6

[edg2s]

Also prefer single-line body-less style when possible.

Fixes #572

[edg2s]

cc @matmarex

[Krinkle]

If there isn't one already, I suggest adding a QUnit.test() case that involves this.something (e.g. from a beforeEach hook), to make sure it neither warns about nor tries to "fix" those. Or, I suppose, really in any input file, it doesn't have to be a QUnit test per-se.

[edg2s]

prefer-arrow-callback is conservative and the autofix is designed to never introduce an error, e.g. if this or arguments is present in the function body:

https://eslint.org/docs/latest/rules/prefer-arrow-callback#:~:text=unbound%20function%20expression

[edg2s]

Added https://github.com/wikimedia/eslint-config-wikimedia/pull/573/files#diff-86bcbe79dd5ebf49dae82bbc6b83b36dfacc16943ed661dd6d319337a93007a0