Make user keymaps fully declarative

This removes the need for the user to define... most things, honestly.
Notably, `main()` is no longer the end user's responsibility. This also
allows us to do fun stuff going forward like validating keymaps for
sanity (ex: the key assigned to `KC_MO(x)` should be assigned to
`KC_TRNS` on the target layer or the user will never be able to escape
that layer).

This also disambiguates `BOARD` to always refer to an actual slab of
silicon, renaming to `USER_KEYMAP`.

Entrypoints are now a bit more wild, and mostly-unsupported boards no
longer have working entrypoints. It's probably just time to scrap those
boards for now (until we have BLE HID and/or bitbang USB HID, at least).

setup.cfg

@@ -3,7 +3,7 @@ exclude = .git,__pycache__,vendor,.venv
 max_line_length = 99
 ignore = X100
 per-file-ignores =
-	boards/**/*.py: E501
+	user_keymaps/**/*.py: F401,E501
 
 [isort]
 known_third_party = analogio,bitbangio,bleio,board,busio,digitalio,framebuf,gamepad,gc,microcontroller,micropython,pulseio,pyb,pydux,uio,ubluepy,machine,pyb