Drop in-browser mode.

[mantaroh]

Is your feature request related to a problem? Please describe.

The #179 will support web containers. This web containers replace the browser mode (#139).
We can drop this browser mode.

Describe the solution you'd like

Before dropping browser mode, we need to implement the followings:

• Support building component files
  • Spear doesn't generate component files. Now spear ignore "components" directory. If user want to component preview feature, we need to build component files.

Describe alternatives you've considered

N/A

Additional context

N/A