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.

[[Catégorie:Salut à Toi]]
[[Catégorie:documentation développeur]]

= Overview =
This is an autogenerated doc for SàT bridge's API
= Signals =
##SIGNALS_PART##
= Methods =
##METHODS_PART##
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