fix(publish): ignore unnecessary files when publishing to npm

[erikian]

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• The changes are appropriately documented (if applicable).
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

Currently, a large number of source/development files (.ts/.map/etc.) from most packages is being published to npm.

The plugin-webpack package is even publishing its entire test folder with a test app, including its node_modules:

This PR addresses the problem by writing a .npmignore file to each package folder before publishing, and then removing the file afterwards. .ts (except .d.ts type definitions), tsconfig.json, .map and .tsbuildinfo files, as well as any /test folders, are now ignored.

Since the .gitignore in the project root has .npmignore, I thought writing and then deleting the files would be an acceptable approach (not to mention easier to review), but actually adding the .npmignore files to Git would be trivial if necessary.