Original code by Evan Czaplicki

Issue #3234

Closes #3295

Since May 5th. `async` and `await` are Python3 keywords.
cf https://www.python.org/dev/peps/pep-0492/

The new version of Python is still in beta, but start to get some use.
And getting the correct highlighting out will help people not use
`async` and `await` as variable name.

Matches color for `cm-link` style

Issue #3274

It can, if the selection is changed in that narrow window, cause
bogus deletion of the selection.

Issue #3273

Closes #3276

https://github.com/codemirror/CodeMirror/issues/3268

And a few minimal tests for the mode

Closes #3258

Enable it for the languages that have switch.

Issue #3252

Issue #3253

Issue #3248

Closes #3244

Issue #3243