We currently check `isPath` and `isString` on crxPath and version
respectively, which is
1. pointless because the module system already does such checks, and
2. wrong because isPath means path literal; a derivation therefore is
not a path.
Brave Browser is a chromium-based browser, too.
+ it use the same web store with Chromium and Google Chrome.
+ the machanism of installing extensions works, and it's verified on
my macOS box.