chore(scripts): use variant index for browser/native instead of individual files

[kuhe]

Issue #, if available:
#1980

Description of changes:

This converts the core submodule browser and native bundler instructions metadata from file-based variants to variant indexes.

This enables:

• far fewer files need to exist in dist-cjs, because now Node.js can use a fully inlined file, and browser/native has a separate but also fully inlined file. The amount of duplication this causes is very small relative to the init/file-count penalty otherwise carried by having so many individual variant files.
• broader bundler support. Now core's package.json exports and browser metadata fields can both point the bundler(s) at the correct entry points, rather than relying solely on the browser metadata field, hoping that the bundler understands a mix of conditional exports and file replacement directives.
• no more false file directives to indicate a file should not be imported. The variant index simply doesn't import the files in the first place (e.g. shared-ini-file-loader).