runtime(java): Recognise string templates (#14150)

As this is encouraged in the referenced JEPs, "to visually
distinguish a string template from a string literal, and
a text block template from a text block", the default
colours for java\%[Debug]StrTempl are made distinct from
java\%[Debug]String.

According to §3.2 Lexical Translations (JLS, c. 1996 or any
more recent version), line terminators, white space, and
comments are discarded before tokens are accepted. Since
a template expression comprises a template processor, a dot,
and a template, it may be visually appealing to break up
its head across a few lines whenever its tail already spans
multiple lines.  Curiously, no allowance for it is made in
the distributed tests for OpenJDK 21; the proposed regexp
patterns take in consideration a line terminator and white
space after a dot.

References:
https://openjdk.org/jeps/430 (Preview)
https://openjdk.org/jeps/459 (Second Preview)
https://openjdk.org/jeps/465

Signed-off-by: Aliaksei Budavei <0x000c70@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/syntax/testdir/input/java_string.java

@@ -1,4 +1,4 @@
-class StringTests
+class StringTests	// JDK 21+ (--enable-preview --release 21).
 {
 	static {
 		String s1 = "A quick brown fox jumps over the lazy dog";
@@ -40,5 +40,65 @@ class StringTests
 		""\"""\u005c\u0022
 		""\"""\""
 		""\"""\""\"""");
+
+		System.out.println(STR."""
+		"
+		\{"\"\""}
+		\{"\"\""}\{"\u005c\u0022"}
+		\{"\"\""}\{"\u005c\u0022"}"
+		\{"\"\""}\"\{"\u005c\u0022\u005c\u0022"}
+		\{"\"\""}\"\{"\"\""}\{"\u005c\u0022"}
+		\{"\"\""}\"\{"\"\""}\""
+		\{"\"\""}\"\{"\"\""}\""\"""");		// JDK 21+.
+
+		String woof = "Woof", dog = "dog", fox = "fox";
+
+		String s6 = STR
+			."A quick brown \{fox} jumps over the lazy \{dog}";
+		String s7 = STR.process(StringTemplate.RAW
+			."\"\{woof}\s!\"");
+		String s8 = STR."""
+			A\s\
+			quick \
+			brown\s\
+			\{fox} \
+			jumps\s\
+			over \
+			the\s\
+			lazy \
+			\{dog}""";
+		String s9 = STR.process(StringTemplate.RAW
+			.
+			"""
+			"\{woof}\s!\"""");
+		String s10 = java.util.FormatProcessor.FMT
+			. "%-14s\{"A\s" + STR . "quick" + "brown"}%s\{fox} "
+			+ java.util.FormatProcessor.FMT
+			. "%-20s\{"jumps\sover the\s"
+					+ STR . "lazy"}%s\{dog}";
+		String s11 = STR."""
+			\"\{			// A nested comment.
+		(new java.util.function.Function<String, String>() {
+			public String apply(String bay) { return bay; };
+		}).apply(woof)
+			}\s!\"""";
+		String s12 = java.util.FormatProcessor.FMT
+			."""
+			%-14s\{STR."""
+				A\s\
+				\{ "quick" } \
+				brown"""}\
+			%s\{ fox } \
+			%-20s\{STR."""
+				jumps\s\
+				over \
+				the\s\
+				\{ "lazy" } """}\
+			%s\{ dog }""";
+		String s13 = STR
+			."\"\{			/* A nested comment. */
+		((java.util.function.Function<String, String>) bay -> bay)
+							.apply(woof)
+			}\s!\"";
 	}
 }