Replace fs-extra.copy by recursively creating directories and files

[paulshryock]

Summary

As of Node v20.8.0, fs.promises.cp is considered Experimental with Stability: 1. For this reason, we're instead using fs-extra.copy. But this dependency adds a lot of extra code to the compiled bundle. Once fs.promises.cp stabilizes, let's switch over.

• https://nodejs.org/dist/latest-v20.x/docs/api/fs.html#fspromisescpsrc-dest-options
• Stabilize fs.cp(), fs.cpSync(), fsPromises.cp() methods nodejs/node#44598

Benefit

• Compiled bundle with fs-extra.copy: 54.1kb
• Compiled bundle without fs-extra.copy: 4.7kb
• Savings: 49.4kb (over 91%)

Acceptance criteria

• GIVEN fs.promises.cp is used instead of fs-extra.copy,
• WHEN this package is compiled with npm run compile,
• THEN the fs-extra dependency is not included in the compiled bundle
  • AND all tests still pass.

[paulshryock]

We're using mock-fs to mock the file system in tests, which does not support fs.promises.cp, and most likely never will.

This seems to be the only thing holding us back from using fs.promises.cp, which actually feels pretty stable at this point, despite the current Experimental status.

Since Node does not natively provide a way to mock the file system, we'll continue to use mock-fs to accomplish this in tests. And for that reason, let's just recursively create directories and files manually, instead of using fs.promises.cp.

If the landscape changes later, we can always revisit.

[paulshryock]

New acceptance criteria

• GIVEN FileSystem.copy is accomplished by recursively creating directories and files instead of fs-extra.copy,
• WHEN this package is compiled with npm run compile,
• THEN the fs-extra dependency is not included in the compiled bundle
  • AND all tests still pass.