Add Deno.gitignore

[mvanhorn]

Reasons for making this change

Deno is a JavaScript/TypeScript runtime with 100k+ GitHub stars. The existing Node.gitignore covers Node.js artifacts but none of Deno's unique patterns.

This addresses @wirecat's feedback on #4101, which asked for a version trimmed to Deno-specific rules only. The original PR (#4101, open since 2022) included IDE and OS rules that don't belong. The author hasn't responded since 2023. This is a fresh, focused replacement.

Included patterns:

• .deno/ - local cache directory when using project-local DENO_DIR
• coverage/ and *.lcov - output from deno coverage

Intentionally NOT ignored:

• deno.lock - Deno documentation says to commit the lockfile for reproducible builds
• vendor/ - Deno documentation says to commit vendored dependencies for hermetic builds

The coverage/ pattern matches what Deno's own standard library uses.

Links to documentation supporting these rule changes

• Deno docs - DENO_DIR setup documents the .deno/ cache directory
• Deno docs - Coverage CLI documents coverage/ output
• Deno docs - Configuration documents that deno.lock should be committed
• Deno standard library .gitignore uses /coverage pattern

If this is a new template

Link to application or project's homepage: https://deno.com/

Merge and Approval Steps

• Confirm that you've read the contribution guidelines and ensured your PR aligns
• Ensure CI is passing
• Get a review and Approval from one of the maintainers

This contribution was developed with AI assistance (Claude Code).