code_block [nfc]: Make switch on CodeBlockSpanType exhaustive

The dark-theme styles, which we'll add soon, have something for all
of `CodeBlockSpanType`'s values.

While we're at it, write the TODO(zulip#754) to update the light-theme
styles. (Hopefully to something that also has a style for all of
`CodeBlockSpanType`'s values.)

Author: chrisbobbe