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>

runtime/syntax/testdir/input/python_ellipsis.py

@@ -0,0 +1,43 @@
+# Ellipsis Literal
+# https://docs.python.org/3/library/constants.html#Ellipsis
+
+# Placeholders
+...
+	...
+x = ...
+y = ... # Comment
+class C: ...
+lambda: ...
+
+# Annotations
+numbers: Tuple[int, ...]
+
+# Doctests
+"""A doctest
+
+>>> class A:
+...     def __init__(self):
+...		...
+>>> class B: ...
+>>> x = ...
+>>> raise ValueError('multi\n    line\ndetail')
+Traceback (most recent call last):
+    ...
+ValueError: multi
+    line
+detail
+>>> print(list(range(20)))  # doctest: +ELLIPSIS
+[0, 1, ..., 18, 19]
+>>> exec(s)  #doctest: +ELLIPSIS
+-3.21716034272e-0...7
+"""
+
+class C:
+	"""
+	>>> class C:
+	...	def __init__(self):
+	...		...
+	"""
+
+# Numpy
+x[..., 0]