By forwarding the task to find the `Plain Text` syntax to `assets`. Not only does
the code become simpler; we also get rid of a call to `self.get_syntax_set()`
which is beneficial to the long term goal of replacing `syntaxes.bin` with
`minimal_syntaxes.bin`.
Note that the use of `.expect()` is not a regression in error handling. It was
previously hidden in `.find_syntax_plain_text()`.