Apply babel to .mjs files

[developit]

Noticed this - seems like .mjs files are included for module resolution, but weren't being transpiled by Babel.

[timneutkens]

Should we also allow pages ending in .mjs 🤔

[developit]

@timneutkens ooh, good point. IMO any case where js and jsx are allowed it seems logical to allow all 3.

[sheerun]

Unfortunately this doesn't work when .mjs are inside node_modules, because such files are excluded by exclude block..

          exclude: (path) => {
            if (/next-server[\\/]dist[\\/]lib/.exec(path)) {
              return false
            }
            return /node_modules/.exec(path)
          },