Fix npm test scripts on Windows

[cedar323]

Summary

• replace the POSIX-only rm -rf src clean command with a small Node cleanup script
• use a double-quoted Prettier glob so Windows npm shells pass the TypeScript file pattern correctly
• keep the build output and generated sources unchanged

Verification

• Reproduced the original Windows failure: npm test stopped at rm -rf src with 'rm' is not recognized
• After the change, npm test passes on Windows: build, format check, lint, and all 383 tape assertions
• git diff --check reports no patch whitespace errors; Git only warns about local CRLF normalization

[MyTH-zyxeon]

Maintainer-facing review note for this Bitgesell maintenance PR:

• Scope is tightly limited to package.json scripts plus a new util/clean.js; no dependency, wallet, address, mnemonic, or runtime library behavior changes are introduced by the diff.
• The clean script change removes the POSIX-only rm -rf src command and uses Node filesystem APIs instead, which matches the PR goal of making npm test usable on Windows npm shells.
• The Prettier glob switch from single to double quotes should preserve the literal ts_src/**/*.ts pattern for Prettier while avoiding the common Windows quoting failure.
• Compatibility note: current Node versions take the fs.rmSync(..., { recursive: true, force: true }) branch; the fs.rmdirSync(..., { recursive: true }) fallback is deprecated on newer Node but only used when fs.rmSync is unavailable.

From the public diff/API state, this looks mergeable (mergeable_state=clean) and low risk after maintainer CI or a focused npm test confirmation.