Report adjacent strings in if-elements and for-elements

[srawlins]

Description

This enhances no_adjacent_string_in_list to include if-elements and for-elements (see tests for examples).

Note that this will trigger on if-elements and for-elements in sets as well 😲 . The lint broadly does not cover sets. Should it? New lint rule? Deprecate this one and make a new lint rule for collection literals and collection elements?

Fixes dart-lang/sdk#58273

Also fixes some grammar in the docs, and makes the test code more idiomatic.

[coveralls]

Coverage increased (+0.007%) to 94.027% when pulling 0207594 on srawlins:fix-2343 into 82e4f0e on dart-lang:master.

[bwilkerson]

The purpose for the lint is to catch missing commas. We can't have the same problem in maps that we do in lists and sets, but yes I think it makes sense to catch these in sets as well.

Either way this is a breaking change, but it's a bigger breaking change if we catch this in sets. I don't know what we want to do about sets (add them or add a new lint), but I do believe that we either need to not report a lint in if and for elements in sets or we need to handle all elements in sets. In its current form this rule is inconsistent.

It probably makes the most sense to limit the behavior to lists for now and open an issue for community feedback on the idea of adding support for sets.

[srawlins]

Filed an issue: dart-lang/sdk#58437

[srawlins]

Replaced by #3741, as I don't use the source repo found in this PR, any longer.