String and math fixes

- Added missing static String constructors
- Implemented String operator for math types
- Added XYZ and YXZ euler angles methods
- Fixed wrong det checks in Basis
- Fixed operator Quat in Basis

include/core/String.hpp

@@ -37,6 +37,14 @@ public:
 
 	~String();
 
+	static String num(double p_num, int p_decimals = -1);
+	static String num_scientific(double p_num);
+	static String num_real(double p_num);
+	static String num_int64(int64_t p_num, int base = 10, bool capitalize_hex = false);
+	static String chr(godot_char_type p_char);
+	static String md5(const uint8_t *p_md5);
+	static String hex_encode_buffer(const uint8_t *p_buffer, int p_len);
+
 	wchar_t &operator[](const int idx);
 	wchar_t operator[](const int idx) const;