runtime(python): highlight ellipsis literals

runtime(python): highlight ellipsis literals

The ellipsis literal (`...`) can be used in multiple contexts:

- Placeholders:     `class Foo: ...`
- Containers:       `Tuple[int, ...]`
- Assignments:      `x = ...`

This is a trickier pattern to match because we can't rely on keyword
boundaries, so we instead look for exactly three dots (`...`).

This does mean that we will match the `...` portion of `x...x`, which
isn't valid Python syntax, but I think that's an acceptable trade-off
that avoids making this pattern much more complex.

Reference:
- https://docs.python.org/3/library/constants.html#Ellipsis

closes: #18107

Signed-off-by: Jon Parise <jon@indelible.org>
Signed-off-by: Christian Brabandt <cb@256bit.org>