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_methods_style_signature.java

@@ -0,0 +1,82 @@
+// VIM_TEST_SETUP let g:java_highlight_functions = 'style'
+// VIM_TEST_SETUP let g:java_highlight_signature = 1
+// VIM_TEST_SETUP set encoding=utf-8 termencoding=utf-8
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Target;
+
+abstract class Style$MethodsTests
+{
+	// TYPES.
+	record Τʬ<α>(α a) { }
+
+	enum E
+	{
+		A("a"), B("b"), C("c"), D("d"),
+		E("e"), F("f"), G("g"), H("h");
+		final String s;
+		private E(String s) { this.s = s; }
+	}
+
+	@Target({ElementType.METHOD, ElementType.CONSTRUCTOR})
+	@java.lang.annotation.Repeatable(Tɐggablɘs.class)
+	@interface Tɐggablɘ
+	{
+		String[] value() default "";
+	}
+
+	@Target({ElementType.METHOD, ElementType.CONSTRUCTOR})
+	@interface Tɐggablɘs
+	{
+		Tɐggablɘ[] value();
+	}
+
+	interface Stylable<Α>
+	{
+		default void ascii$0_() { }
+		default Α μʭʭ$0_() { return null; }
+	}
+
+	// FIELDS.
+	private static final Class<?> CLASS_LOCK = classLock();
+
+	private final Object instanceLock = new Object();
+
+	// CONSTRUCTORS.
+	@Tɐggablɘ @Tɐggablɘ protected Style$MethodsTests() { }
+	<T extends Comparable<T>> Style$MethodsTests(T t, Void v) { }
+	private <T extends Comparable<T>> Style$MethodsTests(T t) { }
+
+	// METHODS.
+	@Tɐggablɘ @Tɐggablɘ abstract void ascii$0_(////////////////
+								);
+	@Tɐggablɘ @Tɐggablɘ abstract <α, β> Τʬ<α> μʭʭ$0_(
+				@SuppressWarnings("bespoke") β b);
+
+	@Tɐggablɘ private native void ascii$1_(/*////////////*/);
+	@Tɐggablɘ private native <α, β> Τʬ<α>[] μʭʭ$1_(
+			java.util.function.Function<β, Τʬ<α>[]> ƒ);
+
+	void Ascii$2_() { }
+	<T, U extends Stylable<T>> void Μʭʭ$2_(U u) { }
+
+	static final native synchronized void ascii$98_();
+	static final native synchronized <α, β> Τʬ<α>[][] μʭʭ$98_(
+			java.util.function.Function<β, Τʬ<α>[][]> ƒ);
+
+	@SuppressWarnings("strictfp")
+	protected static final synchronized strictfp void ascii$99_()
+	{ ascii$98_(); }
+
+	@SuppressWarnings("strictfp")
+	protected static final synchronized strictfp <α, β> Τʬ<α>[] μʭʭ$99_(
+			java.util.function.Function<β, Τʬ<α>[][]> ƒ)
+	{
+		return
+	Style$MethodsTests.<α, β>μʭʭ$98_(ƒ)[0];
+	}
+
+	public static Class<?> classLock() { return Style$MethodsTests.class; }
+
+	@Override @SuppressWarnings("cast")
+	public String toString() { return (String) "Style$MethodsTests"; }
+}