Move all build scripts in `napi` packages into a `scripts` dir. These packages don't have root directories with tons of files like `napi/parser` and `apps/oxlint` do, but IMO it's preferable to have a consistent directory structure in all NAPI packages.
Also, rename these scripts from `.mjs` to `.js`, since these packages are ESM.
Now that `apps/oxlint` and `napi/parser` are both ESM packages (#13723 and #14042), we can use `.js` file extensions for all ESM files, rather than the mix of `.js` and `.mjs` file we had previously.
It's less confusing having to remember what's `.js` and what's `.mjs`, and avoids ugly workarounds like #14038 (`.d.mts`??).