feat(builder): streamline generated template

[mataha]

What's the problem this PR addresses?

The template generated by @yarnpkg/builder's builder new plugin could be better in providing out-of-the-box setup for scaffolding new plugins, specifically on Windows systems with respect to proper file generation. This PR combines slight improvements I imagine a package developer would find welcome.

How did you fix it?

In @yarnpkg/builder:

• create plugin source script file from all segments properly (Windows)
• make default template create a .gitignore containing a bundles directory entry
• mark generated package as private to avoid accidental publish in public registries
• sort dependencies and devDependencies properly (as typings have no business being specified as regular dependencies)
• include package.json scripts for building a development (i.e. non-minified) bundle and cleaning up the generated workspace

Checklist

• I have read the Contributing Guide.

• I have set the packages that need to be released for my changes to be effective.

• I will check that all automated PR checks pass before the PR gets reviewed.

[arcanis]

Those make sense, thanks!

[merceyz]

create plugin source script file from all segments properly (Windows)

It already was correct, ppath is POSIX paths and xfs.writeFilePromise coverts it back to a win32 path if required.

[mataha]

create plugin source script file from all segments properly (Windows)

It already was correct, ppath is POSIX paths and xfs.writeFilePromise coverts it back to a win32 path if required.

In this case I apologize - I come from a Windows environment, thus I panic whenever I see hardcoded paths.