### Motivation
* We have no recommended way to force subprocess command be
(un)threadable
* #4214
* #2119
* #5003
* It's interesting opportunity to have a way to modify specs and CP
using `SpecModifierAlias`.
### Before
```xsh
!(ssh host -T "echo 1")
# output='' # EXPECTED: 1
__xonsh__.commands_cache.threadable_predictors['ssh'] = lambda *a, **kw: True
!(ssh host -T "echo 1")
```
### After
```xsh
xthread
# Mark command as threadable.
!(xthread ssh host -T "echo 1")
# output='1'
```
Closes:
* Closes#4214
* Closes#2119
* Partially closes#5003
Implementation of `SpecModifierAlias` will help in:
* #2618
JFYI #5413
## For community
⬇️ **Please click the 👍 reaction instead of leaving a `+1` or 👍
comment**
---------
Co-authored-by: a <1@1.1>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Gil Forsyth <gforsyth@users.noreply.github.com>
