Add open docs functionality to lints

[FMorschel]

Similarly to Dart-Code/Dart-Code#2785 (adds this functionality to pub packages) and related to the work at #57034, I'd say it would be great if lints had the same functionality to open the docs for them from inside the IDEs.

I'd like to take a look at the work done for Dart-Code/Dart-Code#2785 (at a31be9b) and implement this feature.

CC: @pq @DanTup

[FMorschel]

@bwilkerson wrote Dart-Code/Dart-Code#5405 (comment):

[...]
The change that Parker, Phil, Marya and I have been working toward is to rework some of the documentation for the lints so that

1. there is a page that helps the user understand which lints they might want to enable based on the kinds of problems they want to catch (to improve discoverability),

2. each lint has a description of what the rule does and when a user might want to enable it (to help users understand whether they want to enable the lint), and

3. when a rule produces a diagnostic it has a link to documentation about the diagnostic on https://dart.dev/tools/diagnostic-messages (to help users understand how to fix the problem).

The third step is partially done and ongoing, but the other two steps we're still laying the groundwork for.
[...]

Only because it might be helpful to have informed here too.

[pq]

I would absolutely love to see better integration of linter docs in the IDE. Thanks for taking an interest in this!

[FMorschel]

Here is the CL https://dart-review.googlesource.com/c/sdk/+/411280

[FMorschel]

Ping @bwilkerson for taking a look at the CL. Thanks!

[FMorschel]

Hey @bwilkerson and @pq, I've done all requests commented on the CL. Can I get reviews for this please? Thanks a lot!