Add an input example to gpui (#13534)

Add a single-line text input example to gpui

(I'm hoping to be able to debug keyboard issues without rebuilding the
whole
app every time)

Release Notes:

- N/A

crates/gpui/Cargo.toml

@@ -80,6 +80,7 @@ backtrace = "0.3"
 collections = { workspace = true, features = ["test-support"] }
 util = { workspace = true, features = ["test-support"] }
 http = { workspace = true, features = ["test-support"] }
+unicode-segmentation.workspace = true
 
 [build-dependencies]
 embed-resource = "2.4"
@@ -157,3 +158,7 @@ path = "examples/image/image.rs"
 [[example]]
 name = "set_menus"
 path = "examples/set_menus.rs"
+
+[[example]]
+name = "input"
+path = "examples/input.rs"