Add support for using a language server with multiple languages (#10293)

This PR updates the `extension.toml` to allow specifying multiple
languages for a language server to work with.

The `languages` field takes precedence over `language`. In the future
the `language` field will be removed.

As part of this, the Emmet extension has been extended with support for
PHP and ERB.

Release Notes:

- N/A

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Co-authored-by: Max <max@zed.dev>