Deprecate `\e` escape sequence

This was deprecated in v0.33.0-rc2 in favor of the new `\u{...}`
sequences. For now, deprecating it just means highlighting it as a
warning.

queries/highlights.scm

@@ -61,6 +61,10 @@
 
 ; Literals
 (string) @string
+((escape_sequence) @warning
+ ; Deprecated in v0.33.0-rc2:
+ (#eq? @warning "\\e"))
+(escape_sequence) @string.escape
 (bit_string_segment_option) @function.builtin
 (integer) @number
 (float) @number

test/highlight/constants.gleam

@@ -2,3 +2,9 @@ const f = 100.001e523
 //         ^ number
 //               ^ number
 //                 ^ number
+
+const s = "Hello, \e\t\n"
+//                ^ warning
+//                 ^ warning
+//                   ^ string.escape
+//                    ^ string.escape