[ImageFont] Fix escape sequence parsing, add note to the docs.

doc/classes/ResourceImporterImageFont.xml

@@ -40,7 +40,7 @@
 		</member>
 		<member name="kerning_pairs" type="PackedStringArray" setter="" getter="" default="PackedStringArray()">
 			Kerning pairs for the font. Kerning pair adjust the spacing between two characters.
-			Each string consist of three space separated values: "from" string, "to" string and integer offset. Each combination form the two string for a kerning pair, e.g, [code]ab cd -3[/code] will create kerning pairs [code]ac[/code], [code]ad[/code], [code]bc[/code], and [code]bd[/code] with offset [code]-3[/code].
+			Each string consist of three space separated values: "from" string, "to" string and integer offset. Each combination form the two string for a kerning pair, e.g, [code]ab cd -3[/code] will create kerning pairs [code]ac[/code], [code]ad[/code], [code]bc[/code], and [code]bd[/code] with offset [code]-3[/code]. [code]\uXXXX[/code] escape sequences can be used to add Unicode characters.
 		</member>
 		<member name="rows" type="int" setter="" getter="" default="1">
 			Number of rows in the font image. See also [member columns].

editor/import/resource_importer_imagefont.cpp

@@ -293,18 +293,20 @@ Error ResourceImporterImageFont::import(const String &p_source_file, const Strin
 		}
 		String from_tokens;
 		for (int i = 0; i < kp_tokens[0].length(); i++) {
-			if (i <= kp_tokens[0].length() - 6 && kp_tokens[0][i] == '\\' && kp_tokens[0][i + 1] == 'u') {
+			if (i <= kp_tokens[0].length() - 6 && kp_tokens[0][i] == '\\' && kp_tokens[0][i + 1] == 'u' && is_hex_digit(kp_tokens[0][i + 2]) && is_hex_digit(kp_tokens[0][i + 3]) && is_hex_digit(kp_tokens[0][i + 4]) && is_hex_digit(kp_tokens[0][i + 5])) {
 				char32_t charcode = kp_tokens[0].substr(i + 2, 4).hex_to_int();
 				from_tokens += charcode;
+				i += 5;
 			} else {
 				from_tokens += kp_tokens[0][i];
 			}
 		}
 		String to_tokens;
 		for (int i = 0; i < kp_tokens[1].length(); i++) {
-			if (i <= kp_tokens[1].length() - 6 && kp_tokens[1][i] == '\\' && kp_tokens[1][i + 1] == 'u') {
+			if (i <= kp_tokens[1].length() - 6 && kp_tokens[1][i] == '\\' && kp_tokens[1][i + 1] == 'u' && is_hex_digit(kp_tokens[1][i + 2]) && is_hex_digit(kp_tokens[1][i + 3]) && is_hex_digit(kp_tokens[1][i + 4]) && is_hex_digit(kp_tokens[1][i + 5])) {
 				char32_t charcode = kp_tokens[1].substr(i + 2, 4).hex_to_int();
 				to_tokens += charcode;
+				i += 5;
 			} else {
 				to_tokens += kp_tokens[1][i];
 			}