GDScript: Allow classes declaration to be done in single line

Incidentally, allow multiple statements in single line functions when
using semicolon as a terminator.

modules/gdscript/tests/scripts/parser/features/semicolon_as_terminator.gd

@@ -0,0 +1,22 @@
+#GDTEST_OK
+
+func test():
+	a();
+	b();
+	c();
+	d();
+	e();
+
+func a(): print("a");
+
+func b(): print("b1"); print("b2")
+
+func c(): print("c1"); print("c2");
+
+func d():
+	print("d1");
+	print("d2")
+
+func e():
+	print("e1");
+	print("e2");

modules/gdscript/tests/scripts/parser/features/semicolon_as_terminator.out

@@ -0,0 +1,10 @@
+GDTEST_OK
+a
+b1
+b2
+c1
+c2
+d1
+d2
+e1
+e2

modules/gdscript/tests/scripts/parser/features/single_line_declaration.gd

@@ -0,0 +1,11 @@
+#GDTEST_OK
+
+func test(): C.new().test("Ok"); test2()
+
+func test2(): print("Ok 2")
+
+class A: pass
+
+class B extends RefCounted: pass
+
+class C extends RefCounted: func test(x): print(x)

modules/gdscript/tests/scripts/parser/features/single_line_declaration.out

@@ -0,0 +1,3 @@
+GDTEST_OK
+Ok
+Ok 2