Update highlights.scm for content_tag

[NullVoxPopuli]

Updates the surrounding tokens for content-tag, <template> (the entirety of the contents here are passed to a sub-language, glimmer in this case in the screenshots)

Before:

After:

Info on content_tag:

• Rust Parser: https://github.com/embroider-build/content-tag/
• Spec: https://wycats.github.io/polaris-sketchwork/content-tag.html

Checklist

• All tests pass in CI.
• There are sufficient tests for the new fix/feature.
• Grammar rules have not been renamed unless absolutely necessary.
• The conflicts section hasn't grown too much.
• The parser size hasn't grown too much (check the value of STATE_COUNT in src/parser.c).

[amaanq]

tbh that's a nvim-treesitter specific capture, not a generic upstream one, but it's fine

[NullVoxPopuli]

thanks, I'll PR to nvim-treesitter.