runtime(vim): Update base-syntax, improve function definition highlighting (#14203)

Improve function definition highlighting.

- Match bang and function modifiers - abort etc.
- Only match valid scope modifiers.
- Match listing commands.
- Don't match ex commands in function names.
- Split function syntax groups into :func and :def subgroups.
- Match Vim9-script parameter and return types.
- Limit legacy-script and Vim9-script comments to :func and :def
  definitions, respectively.

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

runtime/syntax/testdir/input/vim_ex_def_fold.vim

@@ -0,0 +1,126 @@
+" Vim :def command
+" VIM_TEST_SETUP let g:vimsyn_folding = "f" | set fdm=syntax
+
+
+" list
+
+def
+def Foo
+def /Foo.*
+
+def | echo "Foo"
+def " comment
+def Foo | echo "Foo"
+def Foo " comment
+
+
+" definition
+
+" empty definition
+def Foo()
+enddef
+
+# curly-brace names
+def {"F"}oo()
+enddef
+
+def F{"o"}o()
+enddef
+
+def Fo{"o"}()
+enddef
+
+def {"F"}o{"o"}()
+enddef
+
+def {"F"}{"o"}{"o"}()
+enddef
+
+def Foo(): number
+  return 42
+enddef
+
+# trailing whitespace
+def Foo(): number  
+  return 42
+enddef
+
+def Foo() # comment
+enddef
+
+def Foo(): number # comment
+  return 42
+enddef
+
+def! Foo(): number
+  return 42
+enddef
+
+def g:Foo(): number
+  return 42
+enddef
+
+def s:Foo(): number
+  return 42
+enddef
+
+def <SID>Foo(): number
+  return 42
+enddef
+
+def foo#bar#Foo(): number
+  return 42
+enddef
+
+" same name as an Ex command
+def s:ls()
+enddef
+
+
+" return types
+
+def Foo(): void
+enddef
+
+def Foo(): void # comment
+enddef
+
+def Foo(): list<dict<number>>
+enddef
+
+def Foo(): func(dict<list<number>>, func, bool, func(number, list<number>)): bool
+enddef
+
+
+" :enddef trailing
+
+def Foo()
+  # trailing whitespace
+enddef  
+
+def Foo()
+enddef | echo "Foo"
+
+def Foo()
+enddef " comment
+
+
+" parameters
+
+def Foo(x: bool, y = 42, z: string = "zed")
+enddef
+
+def Foo(
+    x: bool,
+    y = 42,
+    z: string = "zed")
+enddef
+
+
+" comments
+
+def Foo()
+  # Vim9-script comment
+  "useless string"
+enddef
+