Add support for extensionAlias configuration

[jraoult]

What problem does this feature solve?

I already added details in another closed issue but the gist of it is:

1. I'm sharing ESM modules written with Typescript between Node and the browser
2. I need to write the local import paths with the file extension .js because that's what Node's resolution algorithm expects
3. I need Webpack (Rspack) to know how to map .js to .ts(x) to support this strategy as well

With Webpack I could have (inspired by this comment webpack/webpack#13252 (comment)):

extensionAlias: {
     // Make webpack try the .tsx and .tsx extensions in case the import
      // doesn't match a .js file. This is necessary to get TS and ESM to work
      // together.
      // cf. https://github.com/webpack/webpack/issues/13252#issuecomment-1170215575
      ".js": [".js", ".ts", ".tsx"],
    },

What does the proposed API of configuration look like?

Just match the Webpack configuration: https://webpack.js.org/configuration/resolve/#resolveextensionalias