Add fixture for declaration transpiler test

[MichaelMitchell-at]

I haven't quite figured out how or what to test, but I figure adding a dummy transpiler based on what we have at Airtable could be helpful.

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Test plan

• Covered by existing test cases

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Test

1 test target passed

Targets

//ts/test:rootdir_works_with_repeated_directory_typecheck_test [k8-fastbuild] 26ms

Total test execution time was 26ms. 135 tests (99.3%) were fully cached saving 10s.

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