Add support for block comments in Markdown configuration

[Rusydy]

This pull request includes a small change to the crates/languages/src/markdown/config.toml file. The change adds block comment syntax for Markdown files.

• crates/languages/src/markdown/config.toml: Added block comment syntax <!-- and --> for Markdown files.

Closes #19797

I have been testing it with the following:

1. create a simple markdown file containing following words

good morning - English
Dzień dobry - polish
おはよう - japanese
صباح الخير - arabic
早安 - Chineses (Traditional)

2. comment using cmd+/ and see if there are any errors

Release Notes:

• Added block comment syntax <!-- and --> for Markdown files.

[zed-industries-bot]

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📖	This PR includes links to the following GitHub Issues: #19797 If this PR aims to close an issue, please include a Closes #ISSUE line at the top of the PR body.

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