[gfm] Stack link tokens in overlay with base tokens.

Added `combineTokens` for overlays that can override default `combine` argument.

- If `combineTokens` is set in overlay's state to `true`, tokens will be combined.
- If `combineTokens` is set in overlay's state to `false`, tokens will not combined.
- Else the previous behavior is matched (depends on `combine`)

Closes #2449