feat: codegen browser-variant bundler aliases
Replace the hardcoded unstable-rethrow alias with a list generated from the
filesystem: scripts/generate-browser-variant-aliases.mjs scans packages/next/src
for `<module>.browser.{ts,tsx}` files that have a base `<module>` sibling and
writes the module list to a generated TypeScript file (consumed by
create-compiler-aliases.ts) and a generated Rust file (consumed by
get_next_client_resolved_map). Both bundlers now alias every such module's
default to its `.browser` variant for the browser bundle.
`pnpm check-browser-variant-aliases` (added to `lint`) fails if the committed
lists are out of date with the filesystem.
