runtime(java): Compose "g:java_highlight_signature" and "g:java_highlight_functions"

With the variables defined, distinctly highlight parts of
a method declaration header: its name and parameter list
parens, from its type parameters, return type, and formal
parameters; and distinctly highlight parts of a lambda
expression: its parameter list parens and the arrow, from
its formal parameters and identifiers.

closes: #15083

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

runtime/syntax/testdir/input/java_annotations_signature.java

@@ -0,0 +1,78 @@
+// VIM_TEST_SETUP let g:java_highlight_functions = 'style'
+// VIM_TEST_SETUP let g:java_highlight_signature = 1
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Target;
+
+class Annotations$Tests
+{
+	@Target(ElementType.TYPE_USE)
+	@interface Tag
+	{
+		String value() default "";
+		String kind() default "";
+	}
+
+	@Target(ElementType.TYPE_USE)
+	@interface Text
+	{
+		String[] value() default {""};
+	}
+
+	@Target({
+		ElementType.METHOD,
+		ElementType.PARAMETER,
+		ElementType.TYPE,
+	})
+	@interface Labels
+	{
+		Label[] value();
+	}
+
+	@java.lang.annotation.Target({
+		java.lang.annotation.ElementType.METHOD,
+		java.lang.annotation.ElementType.PARAMETER,
+		java.lang.annotation.ElementType.TYPE,
+	})
+	@java.lang.annotation.Repeatable(Labels.class)
+	@interface Label
+	{
+		String value() default "";
+		Class<?> type() default Label.class;
+		boolean redundant() default true;
+		Text text() default @Text;
+		Tag head() default @Tag();
+		Tag tail() default @Tag(value = "", kind = "");
+	}
+
+	/* Use identity cast expressions to nest TYPE_USE annotations. */
+								@Label(
+							(@Text({
+		(@Text({ "a", "aa", "aaa", "aaaa", }) String) "as",
+		(@Text({ "b", "bb", "bbb", "bbbb", }) String) "bs",
+		(@Text({ "c", "cc", "ccc", "cccc", }) String) "cs",
+		(@Text({ "d", "dd", "ddd", "dddd", }) String) "ds",
+							}) String) "abcd")
+	interface Primer { }
+
+	@Label @Label() @Label("""
+	n\
+	o\
+	O\
+	p""")
+	@Label(head = @Tag(value = "@Label"/*, kind = "name"*/))
+	@Label(// value = "Method",
+		type = Annotations$Tests.class,
+		redundant = !!!(1 != 1),
+		head = @Tag(value = "@Label"),
+		text = @Text({ "})", "({" }))
+	static void noOp(@Label @Label() @Label("dummy")
+		@Label(head = @Tag(/*value = "@Label",*/ kind = "name"))
+		@Label(// value = "Parameter",
+			type = Annotations$Tests.class,
+			head = @Tag(value = "@Label"),
+			text = @Text({ "){", "}(" }))
+		Object dummy)
+	{
+	}
+}