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.

syntax: glob
*.pyc
*.pyv
*.swp
*.swo
tags
twistd.log
twistd.pid
bridge_constructor/generated
_trial_temp/
sat.egg-info
*.un~
dist
MANIFEST
build
ctags_links/
Session.vim
.build
.pytest_cache