fix(config): reject unknown highlighting fields

[leno23]

Closes #3127.

What changed

• Add deny_unknown_fields to the [markdown.highlighting] config struct so misspelled keys like extra_grammar are reported while parsing config.toml instead of being silently ignored.
• Add a regression test for the singular extra_grammar typo from the issue.

Verification

• cargo fmt --check
• cargo test -p config

Risk

Low. This only tightens validation within the [markdown.highlighting] table and should surface existing typos as config errors rather than changing highlighting behavior for valid configs.

[Keats]

What's the difference with #3131? And on the wrong branch