feat: emit declarations with a tool other than tsc

[alexeagle]

Mirrors how the transpiler attribute works.

Here's a profile, showing that frontend and backend targets are type-checked in parallel with core even though they both depend on it. The small green boxes in the upper-left are the 10ms swc action that produces the core/index.d.ts file needed by the type-checker for frontend and backend

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Changes are visible to end-users: yes

• Searched for relevant documentation and updated as needed: yes/no
• Breaking change (forces users to change their own code or config): yes/no
• Suggested release notes appear below: yes/no

Test plan

• Covered by existing test cases
• New test cases added
• Manual testing; please provide instructions so we can reproduce:

[aspect-workflows]

Test

All tests were cache hits

82 tests (100.0%) were fully cached saving 12s.

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Buildifier      Format

[alexeagle]

@jbedard this is the next step in the PR, to make the SWC-transpiled declarations appear as the types on the terminal rule in the macro produced for core:core. I have to juggle the logic for when to produce a js_library target "tree".

Also before landing this I think the declaration producing rule should be in rules_swc rather than a bzl file here.