Add ability to write importmap into a .js file

[dmail]

Fixes #78

[github-actions]

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Error: Error while trying to collect info after merging output_as_js_file into main.

Error: Command failed: git fetch --no-tags --prune origin output_as_js_file
fatal: couldn't find remote ref output_as_js_file

    at genericNodeError (node:internal/errors:983:15)
    at wrappedFn (node:internal/errors:537:14)
    at ChildProcess.exithandler (node:child_process:414:12)
    at ChildProcess.emit (node:events:524:28)
    at maybeClose (node:internal/child_process:1101:16)
    at Socket.<anonymous> (node:internal/child_process:456:11)
    at Socket.emit (node:events:524:28)
    at Pipe.<anonymous> (node:net:351:12)

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_{Generated by @jsenv/performance-impact during performance_impact#15349793928 on 24c3826}