Fix autofill in Firefox extension

The script injected into addons/firefox/src/page.ts
is built without browserify builtins since it is supposed
to be very lightweight. Recent additions to webui/page_access.ts
which is imported by page.ts brought in a host of additional
dependencies.

page.ts only needs the definitions for the data types exchanged
in RPC messages between the passcards front-end and the page.

Move these types into a separate forms.ts file which has no
external dependencies.