CodeLens text currently hardcodes `text.muted`, so changing its color
also changes unrelated muted UI text such as the sidebar.

This adds a dedicated `editor.code_lens.foreground` theme color. When
omitted, it falls back to `text.muted`, preserving the existing
appearance of themes while allowing CodeLens text to be customized
independently.

Release Notes:

- Added `editor.code_lens.foreground` for customizing CodeLens text
independently.

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Co-authored-by: Kirill Bulatov <mail4score@gmail.com>