Support "instance" pseudo-scope (#1497)

- Implements the "instance" pseudo-scope. It functions grammatical kind
of like a scope, but hoists over ranges and uses its input in its own
way.
- Fixes #29
- Depends on #1496
- See #1490 for some discussion of how we got to this implementation

## Checklist

- [x] Handle word scopes
- [x] I have updated the
[docs](https://github.com/cursorless-dev/cursorless/tree/main/docs) and
[cheatsheet](https://github.com/cursorless-dev/cursorless/tree/main/cursorless-talon/src/cheatsheet)
- [x] Merge with #1496 ?
- [x] check performance on gigantic file
- [x] Come up with story around repeater noises. Maybe interact with
some new knausj quick actions interface, where "instance" establishes
that the "forward" quick action will perform action on additional target
- [x] Document how to add one more selection somewhere: "take that and
next instance that"
- [x] Capture writing from #1490
- [x] "first three instances"
- [x] I have added
[tests](https://www.cursorless.org/docs/contributing/test-case-recorder/)
- [x] Handle narrowing range by selection
- [x] I have not broken the cheatsheet

Author: pokey

src/modifiers/scopes.py

@@ -29,6 +29,7 @@
     "funk name": "functionName",
     "funk": "namedFunction",
     "if state": "ifStatement",
+    "instance": "instance",
     "item": "collectionItem",
     "key": "collectionKey",
     "lambda": "anonymousFunction",