Rename `comment_type` field

[hliberacki]

The comment_type field in SourceDiscoverConfig is misleading - it doesn't configure anything about comment types or styles. It selects the programming language used for parsing.

Current behavior:

[codelinks.projects.my-project.source_discover]
comment_type = "cpp"

It already lists language names (cpp, python, cs, yaml, rust), and later maps them to file extensions. The field effectively means "language".

Proposed change:

• Rename comment_type to language across:
• A deprecation period could accept both comment_type and language, emitting a warning for the old name before removing it in a future release.

[codelinks.projects.my-project.source_discover]
language = "cpp"