@ as __matmul__ operator only on Python 3,

Add tests for Python mode for this use case.

Closes #3957

There were two issues:

1. Github flavored markdown calls Objective C `objective-c`
2. The gfm.js mode didn't include `-` in its fenced code block regex

Additionally, test/index.html did not import the mode/meta.js file,
which prevented mode lookup by name from working in unit tests.

Added tests and fixed some issues uncovered by testing.

 * Support nested block comments.
 * Improve number and operator parsing.
 * Improve string parsing.
 * Style brackets correctly.
 * Improve #-words.

Closes #3840

Closes #3742
Closes #3744

zt was placing 40% of the line above the visible area, and zb was placing it too high.

Which is kind of arbitrary but seems to give less surprising
results.

Issue #3658

The code is now clearer, and will prefer the nearer end of the atomic
range as long as that means there is still progress (i.e. the cursor
does not get stuck).

Issue #3658

Issue #3517

Closes #3245

Closes #3470

Issue #3395

Issue #3362

Issue #3274

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

And a few minimal tests for the mode

Issue #3248

https://github.com/codemirror/CodeMirror/issues/3219

Closes #3172

https://github.com/codemirror/CodeMirror/issues/3205