Add extensions in compiled CTS/MTS

`@hackbg/4mat` is compiled and published using `@hackbg/ubik`. When installed in a Deno project, at the path `node_modules/.deno/@hackbg+4mat@3.1.2/node_modules/@hackbg/4mat/4mat.dist.d.mts` is found the reexport statement:

```ts
export * from './4mat.browser';
```

which should instead be:

```ts
export * from './4mat.browser.dist.mts';
```

because otherwise Deno fails to see the reexported items.