Added `SpecModifier` and ``threadable`` and ``unthreadable`` aliases

[anki-code]

TODO:

[ ] Test proc.threadable [ ] Docs

Before

__xonsh__.commands_cache.threadable_predictors['ssh'] = lambda *a, **kw: True
!(ssh host -T "echo 1")

After

!(threadable ssh host -T "echo 1")

Closes #4214 #2119

For community

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[anki-code]

@gforsyth @jnoortheen I splitted this PR into two:

• This PR has pure refreshing of specs.py without logic changes. Here I just group the code into functions.
• The next PR to consider with positive difference is 5443 - it's branch from this PR.

Please review and merge. Thanks!