runtime(vim): Update base-syntax, improve enum highlighting

Match enum values and missing class keywords.

fixes: #15970

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

runtime/syntax/testdir/input/vim9_ex_enum2.vim

@@ -0,0 +1,32 @@
+vim9script
+# Vim :enum command
+# VIM_TEST_SETUP hi link vim9EnumValue Todo
+
+
+# See: https://github.com/vim/vim/pull/16365#issuecomment-2571420551
+
+enum Letter
+    #
+    #########################################
+    A("(\" # not a comment NOR_ANOTHER_VALUE,
+        \ "), B(")\""), C($"'')
+        \('"), D($'""(),"'), E,
+    F(
+    ")" .. # FA,
+    "(" # FB,
+    ), G
+    #enum NotSupported
+    #endenum
+
+    def new(this.value = v:none)
+        return
+    enddef
+    const value: string
+endenum
+
+for letter in Letter.values
+    echo letter
+endfor
+
+echo Letter.D
+