add the build github actions workflow

[rgl]

this a WIP for the github actions workflow mentioned at #17.

please do not yet merge the code until you review it, especially the TODO parts, where I request your input in order to decide how to proceed with the implementation.

to deploy this you need to:

1. at your npmjs.com account access token page, create a new Automation Classic Token.
2. at your github repository settings, create a new actions repository secret named NPM_TOKEN with the token created at 1.
3. at your github repository settings, create a protected tag for the v* pattern.

to trigger a new non-release build, you need to:

1. push code to the repository.
2. wait for the github actions workflow to build, lint, and create the npm package (will be available as a build artifact).

to trigger a new release build and publish the npm package, you need to:

1. push a tag that starts with a v character, e.g., v1.0.6-pre1.
2. wait for the github actions workflow to build, lint, create the npm package (will be available as a build artifact), and publish the npm package to https://www.npmjs.com/package/whip-whep.

i've done a couple of test builds, which you can see at:

• https://www.npmjs.com/package/@rgl/whip-whep-test-tmp
• https://github.com/rgl/whip-whep-js/actions

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