[parley] new selection logic and an editor example (#106)

Adds a new `Selection` type for dealing with cursor movement and
selection.

Also includes a `vello_editor` example that serves as a testbed for
checking the logic. This supports mouse selections, arrow keys (+ shift
to extend selections), backspace, delete and basic text input.

Marked as draft because the code is a mess but making this public so
people can play with it and report bugs.

.typos.toml

@@ -6,6 +6,16 @@
 # word is treated as always correct. If the value is an empty string, the word
 # is treated as always incorrect.
 
+[default]
+extend-ignore-re = [
+    # Matches lorem ipsum text.
+    # In general, regexes are only matched until the end of a line by typos,
+    # and the repeated matcher at the end of both of these also ensures that
+    # matching ends at quotes or symbols commonly used to terminate comments.
+    "Lorem ipsum [a-zA-Z .,]*",
+    "Phasellus in viverra dolor [a-zA-Z .,]*",
+]
+
 # Match Identifier - Case Sensitive
 [default.extend-identifiers]
 Beng = "Beng"