build(repo): Add build:tarball scripts

[Lms24]

This PR adds npm scripts to build NPM tarballs.
Because rrweb uses release-it for publishing their packages, there was no need to create tarballs before. However, as we're using Craft and we have to put tarballs into the GHA build artifacts for craft to pick them up, we need a script to build them easily.

Furthermore, release-it seems to have some way of adding the LICENSE file to the respective packages. We actually need to copy them into the packages to have them included in the tarballs.

Due to how rrweb set up the prepack hook, whenever yarn build:tarball is executed, the other build steps (rollup, type checking) are executed automatically beforehand. While this means we're probably gonna execute builds twice in CI, I think this is fine as we're not gonna release often (🤞) and we want to keep changes diverging from upstream minimal.

ref #15

[Lms24]

I checked the tarball contents with the official rrweb tarballs and they're almost identical. For some reason, rrweb has tslib in its rrweb/es/ext folder while our tarball doesn't. However, the transpiled files in rrweb/es/packages/rrweb (i.e. the actual rrweb files that require polyfills from tslib) all require tslib from another path (rrweb/es/packages/rrweb/ext/tslib) which is included in both versions of the tarball. So I think we're good in terms of package integrity. Just leaving this here for future reference.