runtime(vim): Update base-syntax file, improve class, enum and interface highlighting

- Enable folding of class, enum and interface declarations.
- Highlight constructor names with the Function highlight group, like
  other special methods.
- Mark function definitions using special method names as errors.
- Highlight :type arguments.

fixes: #14393#issuecomment-2042796198.
closes: #13810

Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/syntax/testdir/input/vim9_ex_special_method_definitions.vim

@@ -0,0 +1,113 @@
+vim9script
+# Vim9 special methods new*(), empty(), len(), string()
+
+def new()
+enddef
+
+def newOther()
+enddef
+
+def newyetanother()
+enddef
+
+def empty(): bool
+  return true
+enddef
+
+def len(): number
+  return 0
+enddef
+
+def string(): string
+  return ""
+enddef
+
+class A
+  def new()
+    def newNested()
+    enddef
+    def empty(): bool
+      return true
+    enddef
+    def len(): number
+      return 0
+    enddef
+    def string(): string
+      return ""
+    enddef
+  enddef
+
+  def newOther()
+    def newNested()
+    enddef
+    def empty(): bool
+      return true
+    enddef
+    def len(): number
+      return 0
+    enddef
+    def string(): string
+      return ""
+    enddef
+  enddef
+
+  def newyetanother()
+    def newNested()
+    enddef
+    def empty(): bool
+      return true
+    enddef
+    def len(): number
+      return 0
+    enddef
+    def string(): string
+      return ""
+    enddef
+  enddef
+
+  def empty(): bool
+    def newNested()
+    enddef
+    def empty(): bool
+      return true
+    enddef
+    def len(): number
+      return 0
+    enddef
+    def string(): string
+      return ""
+    enddef
+    return true
+  enddef
+
+  def len(): number
+    def newNested()
+    enddef
+    def empty(): bool
+	return true
+    enddef
+    def len(): number
+	return 0
+    enddef
+    def string(): string
+	return ""
+    enddef
+    return 0
+  enddef
+
+  def string(): string
+    def newNested()
+    enddef
+    def empty(): bool
+	return true
+    enddef
+    def len(): number
+	return 0
+    enddef
+    def string(): string
+	return ""
+    enddef
+    return ""
+  enddef
+endclass
+