Display deprecated label when @[Deprecated] is used

[bcardiff]

Depends on #7652 (marked as draft until merged)

The last commit of the PR adds support of the doc generator tool to understand the @[Deprecated] and emit the same label that is currently emitted when a # DEPRECATED: ... comment is found.

Even before this PR the labels are only emitted for methods. But the implementation will be ready to handle them once other annotations are preserved by the parser/compiler.

Show me a sample (with screenshot)

Given the following code:

module FOO
  # DEPRECATED: Use `Bar` instead1
  @[Deprecated("Use `Bar` instead2", "sdfs")]
  class Foo
    # DEPRECATED: Use `VAL_B` instead1
    @[Deprecated("Use `VAL_B` instead2")]
    VAL_A = 10
    VAL_B = 20

    # DEPRECATED: Use `#bar` instead1
    @[Deprecated("Use `#bar` instead2")]
    def foo
    end

    # DEPRECATED
    @[Deprecated]
    def foo2
    end

    def bar
    end

    # DEPRECATED: Use `#qux` instead1
    @[Deprecated("Use `#qux` instead2")]
    macro baz
    end

    macro qux
    end
  end

  class Bar
  end
end

This is the generated output:

[straight-shoota]

Injecting annotations into the markdown string looks like a cheap hack.
But should be fine for now.

[bcardiff]

Yes @straight-shoota I agree. It's cheap.

Merging this will mean that the time deprecation does not need to be in the comments 🚀