Add parsing support for Unicode escape sequences

While the lexer already supported Unicode escape sequences, I forgot to
add support for this in the parser. This commit fixes this, so you can
now use strings like this:

    foo\u{AC}bar

Changelog: added

ast/src/parser.rs

@@ -1617,7 +1617,7 @@ impl Parser {
 
                     return Ok(Expression::DoubleString(Box::new(string)));
                 }
-                TokenKind::StringText => {
+                TokenKind::StringText | TokenKind::UnicodeEscape => {
                     values.push(DoubleStringValue::Text(Box::new(
                         self.string_text(token),
                     )));
@@ -1651,7 +1651,10 @@ impl Parser {
         let mut value = start.value;
         let mut end_loc = start.location.clone();
 
-        while self.peek().kind == TokenKind::StringText {
+        while matches!(
+            self.peek().kind,
+            TokenKind::StringText | TokenKind::UnicodeEscape
+        ) {
             let token = self.next();
 
             value += &token.value;
@@ -5915,6 +5918,39 @@ mod tests {
                 location: cols(1, 10)
             }))
         );
+
+        assert_eq!(
+            expr("\"foo\\u{AC}bar\""),
+            Expression::DoubleString(Box::new(DoubleStringLiteral {
+                values: vec![DoubleStringValue::Text(Box::new(StringText {
+                    value: "foo\u{AC}bar".to_string(),
+                    location: location(1..=1, 2..=13)
+                })),],
+                location: location(1..=1, 1..=14)
+            }))
+        );
+
+        assert_eq!(
+            expr("\"foo\\u{AC}\""),
+            Expression::DoubleString(Box::new(DoubleStringLiteral {
+                values: vec![DoubleStringValue::Text(Box::new(StringText {
+                    value: "foo\u{AC}".to_string(),
+                    location: location(1..=1, 2..=10)
+                })),],
+                location: location(1..=1, 1..=11)
+            }))
+        );
+
+        assert_eq!(
+            expr("\"\\u{AC}bar\""),
+            Expression::DoubleString(Box::new(DoubleStringLiteral {
+                values: vec![DoubleStringValue::Text(Box::new(StringText {
+                    value: "\u{AC}bar".to_string(),
+                    location: location(1..=1, 2..=10)
+                })),],
+                location: location(1..=1, 1..=11)
+            }))
+        );
     }
 
     #[test]

docs/source/getting-started/syntax.md

@@ -413,6 +413,13 @@ bar \
 baz" # => "foo bar baz"
 ```
 
+Double quoted strings support Unicode escape sequences using the syntax
+`\u{XXXXX}`, such as this:
+
+```inko
+"foo\u{AC}bar"
+```
+
 ### Integers
 
 The syntax for integers is as follows: