feature: Support comments for dependencies

[pawamoy]

Is your feature request related to a problem? Please describe.

Other things I encountered in my own repositories and elsewhere:

• # TODO: Remove this once MkDocs 1.2 is common enough.
• # TODO: Remove this once https://github.com/owner/repo/pull/5 is merged and released.

First one is about dropping a code branch when a certain version of a dependency is old enough (months, years), or common enough (number of downloads per month?). "Common" enough doesn't sound very robust. Checking the age of a version sounds a bit better.

Second one is kind of the same. We're just earlier than the version release.

Describe the solution you'd like

A new comment kind that expects:

• a dependency name
• a dependency exact version, or version specifier (for example >1.2 to target the version that comes after 1.2)

Ideas:

• # YORE: 2yo MkDocs 1.2: Remove block.
• # YORE: Age MkDocs 1.2: 2 years: Remove block.
• # YORE: Age 2y: MkDocs 1.2: Remove block.
• # YORE: YSR 2: MkDocs 1.2: Remove block. (Years Since Release)
• # YORE: Dep mkdocs==1.2: 2 years: Remove block.
• # YORE: 2ysr of mkdocs==1.2: Remove block. (this one reads better)
• # YORE: 2 years since release of mkdocs==1.2: Remove block. (full words)
• # YORE: 2y into mkdocs==1.2: Remove block. (shorter, as readable)

We can reuse the parser for --warn-before-eol to parse 2y, 3m, etc.

Describe alternatives you've considered

/

Additional context

At some point, maybe it would be interesting to let users develop their own comments and define how they are handled. This would allow extreme flexibility. Like querying GitHub to check if a PR is merged.