Add Deno ecosystem support

[sbs44]

What are you trying to accomplish?

Adds Deno as a new ecosystem, enabling Dependabot to update dependencies in deno.json and deno.jsonc import maps. Parses both jsr: and npm: specifiers (scoped, unscoped, versionless, and subpath) and queries jsr.io and npmjs.org registries directly for version lookups.

Gated behind enable-beta-ecosystems per NEW_ECOSYSTEMS.md.

Closes #2417

Anything you want to highlight for special attention from reviewers?

• No lockfile regeneration yet — this PR updates version constraints in deno.json/deno.jsonc only. deno.lock regeneration (artifact updating) is planned as a follow-up.
• JSONC support — deno.jsonc files can contain comments and trailing commas. The parser strips these before JSON parsing.
• Registry approach — queries jsr.io (/@scope/name/meta.json) and npmjs.org (/name) APIs directly rather than shelling out to the Deno CLI.
• MetadataFinder returns source URLs for npm packages but not jsr packages (jsr.io doesn't expose repository metadata in its API).

How will you know you've accomplished your goal?

45 RSpec tests pass across all components (Version, Requirement, FileFetcher, FileParser, UpdateChecker, FileUpdater, MetadataFinder). Rubocop and Sorbet (typed strict) pass cleanly. Dry-run tested against real Deno projects containing scoped npm packages, versionless specifiers, and subpath imports — all dependencies detected and correct updates proposed.

Checklist

• I have run the complete test suite to ensure all tests and linters pass.
• I have thoroughly tested my code changes to ensure they work as expected, including adding additional tests for new functionality.
• I have written clear and descriptive commit messages.
• I have provided a detailed description of the changes in the pull request, including the problem it addresses, how it fixes the problem, and any relevant details about the implementation.
• I have ensured that the code is well-documented and easy to understand.

[Hajime-san]

• feat(managers): add Deno manager support renovatebot/renovate#37828

I think that you could also refer to the implementation in Renovate, although it hasn't been merged yet.
Renovate also supports workspace and node-compat package.json. It does not include support for private registries via .npmrc currently.

[Decodetalkers]

Seems there are conflicts.. when can it be merged?

[sbs44]

@Decodetalkers I'll rebase now. Re: merge timing, that's up to the maintainers... it looks like a maintainer self-assigned this ~3 weeks ago, but I imagine they're working through an AI-infused backlog and new ecosystems are at the bottom of the pile.

@robaiken – Do you by chance have a timeline on this?

[robaiken]

@sbs44 I don't have an exact timeline to share just yet. Adding a new ecosystem requires a lot of coordination on our end, so it would be good to go over everything together. If you're interested in setting up a meeting, you can reach me at my GitHub username @github.com

[sbs44]

@robaiken Understood. Emailing now to set up a time.

[markhallen]

Good start on adding Deno ecosystem support! I've left some inline comments on potential issues — mostly around edge cases with versionless specifiers, subpath imports, and error handling.