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.

FROM libervia/prosody:e2e

LABEL maintainer="Goffi <tmp_dockerfiles@goffi.org>"

RUN prosodyctl register demo server1.test demo