Rework TextEdit arrow navigation to handle Unicode graphemes (#5812)

* [x] I have followed the instructions in the PR template

Previously, navigating text in `TextEdit` with Ctrl + left/right arrow
would jump inside words that contained combining characters (i.e.
diacritics). This PR introduces new dependency of `unicode-segmentation`
to handle grapheme encoding. The new implementation ignores whitespace
and other separators such as `-` (dash) between words, but respects `_`
(underscore).

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Co-authored-by: lucasmerlin <hi@lucasmerlin.me>

crates/egui/Cargo.toml

@@ -87,6 +87,7 @@ ahash.workspace = true
 bitflags.workspace = true
 nohash-hasher.workspace = true
 profiling.workspace = true
+unicode-segmentation.workspace = true
 
 #! ### Optional dependencies
 accesskit = { workspace = true, optional = true }