plugin text syntaxes: remove side effect on init:
When text syntaxes was initialised, `syntaxes` dict was filled, but it was a class
attribute. As a result, re-initialising it (which may happen for tests, and maybe in the
future dynamically), was not starting with an empty dict, but with an already-filled one,
resulting in conflicts.
This patch fixes it by moving `syntaxes` to an instance attribute.