Added join lines action (#1901)

`"join air"`
`"join block air"`

Fixes #50

## Checklist

- [x] I have added
[tests](https://www.cursorless.org/docs/contributing/test-case-recorder/)
- [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] I have not broken the cheatsheet

---------

Co-authored-by: Pokey Rule <[email protected]>

Author: 2 people

src/spoken_forms.json

@@ -26,6 +26,7 @@
       "hover": "showHover",
       "indent": "indentLine",
       "inspect": "showDebugHover",
+      "join": "joinLines",
       "post": "setSelectionAfter",
       "pour": "editNewLineAfter",
       "pre": "setSelectionBefore",