chore(web): add public API for fetching keyboards without activating them

[jahorton]

OK, that's very side-effecty! We should have a public API for fetching the keyboard.

For now I am okay with using fetchKeyboard here so long as we publish an API before 18.0 stable.

Originally posted by @mcdurdin in keymanapp/help.keyman.com#1272 (comment)

Our help-site keyboard documentation pages operate by fetching the keyboard, then building documentation versions of its OSK - all without formally "activating" the keyboard for use when typing. We've used undocumented, internal functionality to do so until now; it'd be wise to design a 'proper' approach and do that instead.

[mcdurdin]

While this came out of changes to the help site, I think we should consider it as part of the refactoring for loading .kmx keyboards.