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.

version: "3.6"
services:

  backend:
    image: libervia/backend:dev
    build: backend-dev

  web:
    image: libervia/web:dev
    build: libervia-web-dev
    depends_on:
      - backend