Update packages

[cviebrock]

This fixes a few Dependabot vulnerabilities in the package:

1. Most dependencies have been bumped to the latest versions.
2. The brace-expansion package (required by minimatch) is still vulnerable, so we override it to the next patch version which fixes the issue.
3. The postcss-url package hasn't been maintained in several years. We only use it in one place in the code to rebase URLs. This is replaced with the better-maintained postcss-rebase-url package.
4. chokidar has been updated and has a different way to define the watch and ignore paths. This has been updated.
5. globby needed to be imported differently.
6. NEW: pnpm prettier:write script added
7. Moved from "CommonJS" and "node10" in module resolution to "NodeNext". This was required in order to get the postcss-rebase-url package recognized. (TBH, I'm not sure if there are any knock-on effects.)

All the CI checks still pass, and pnpm audit returns no vulnerabilities. 🎉

It has been tested by symlinking the package and build binary over to a working project, and running yarn legaseer. The output looks identical to what it was with the previous version. The --symlink=foo,bar feature works. And updating a .less file in the project triggers a recompile, as expected.