Support determining the page_width given a list / lists of multiple included files

[srawlins]

For dart-lang/sdk#47256, the analyzer recently supports merging multiple include files, in an analysis options file. So before we only supported:

include: some_file.yaml

and we now additionally support:

include:
  - one_file.yaml
  - ../../another.yaml
  - package:foo/thrid.yaml

I believe this package has it's own YAML merging logic to read the single (chain of) include files.

[srawlins]

Notes for whoever implements this in dart_style:

• The "multiple includes in a file" "design" is this comment.
• I don't believe lib/src/analysis_options/merge_options.dart needs to change, just lib/src/analysis_options/analysis_options_file.dart around line 73.
• The implementation in the analyzer is very small (if you ignore whitespace): dart-lang/sdk@52001ff?w=1#diff-3ba3bd2972295aaa78b1f8ee79ffc651e66b869ae6ef69ec19f1d33472e51c7c - Just walk the include list and merge them in the right direction in the right order.

[srawlins]

@munificent would you mind if I took a whack at this?

[munificent]

I was honestly surprised when I first implemented this that only a single include was supported.

@munificent would you mind if I took a whack at this?

Sure, not at all! Thanks!