Implement """triple-quoted""" multi-line strings

Fixes #589

test/asm/multi-line-strings.asm

@@ -0,0 +1,32 @@
+S EQUS "Hello"
+
+PRINTT "\"\"\"\n"
+
+PRINTT """{S}
+world
+"""
+
+PRINTT """The multi-line string \ ; line continuations work
+can contain:
+- "single quotes"
+- ""double quotes""
+- even escaped \"""triple"\"" ""\"quotes\"\"\"
+!"""
+
+PRINTT """\n"""
+
+printarg: MACRO
+	PRINTT "arg <\1>\n"
+	PRINTT """arg (\1)\n"""
+ENDM
+
+	printarg "
+	printarg """
+
+EMPTY1 EQUS ""
+EMPTY2 EQUS "\ ; comment
+"
+EMPTY3 EQUS """"""
+EMPTY4 EQUS """\ ; comment
+"""
+	PRINTT STRCAT("(", "{EMPTY1}", "{EMPTY2}", "{EMPTY3}", "{EMPTY4}", ")\n")