[build_web_compilers] Standardize web app entrypoint selection

[parlough]

Provides a stable order for finding web entrypoints when hot reload is enabled. This fixes an issue we were hitting with Jaspr serving where a test file with main was being preferred over an entrypoint in the web directory. Now the following priority is followed, while still ensuring they are entrypoints:

1. The specified order of directories. So by default: web -> test -> example -> benchmark.
2. Files closer to the root. So considers web/main.dart before web/a/main.dart.
3. Files named main.dart.
4. Files with an earlier alphanumeric value.

This also updates the web-assets-path option to support a YAML list of values rather than just a comma separated list and adds additional validation to the option.

[davidmorgan]

@Markzipan

[Markzipan]

LGTM - with a small change.

Note that the final changes to hot reload webdev-side are being added in this change: dart-lang/webdev#2867

after some DWDS-specific glue is landed in our SDK. This may delay formal hot reload support unless it's been handled separately in Jaspr.

[parlough]

Thanks for taking a look! Updated.

Note the failing test seems to be an unrelated flake that's also timing out on main.

[Markzipan]

LGTM!