runtime(vim): Update base-syntax and generator, only match valid predefined variables

- Only match valid predefined and option variables.
- Match scope dictionaries.
- Highlight scope prefixed variables as a scope dictionary accessor. The
  vimVarScope syntax group can be linked to vimVar to disable this.
- Include support for Neovim-only predefined and option variables.

Temporary collateral damage - scope dictionaries match instead of keys
in dictionary literals.

closes: #16727

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

runtime/syntax/testdir/dumps/vim_variables_17.dump

@@ -17,4 +17,4 @@
 @6|"+0#0000e05&|\| |c|o|m@1|e|n|t| +0#0000000&@58
 @6|\+0#e000e06&| +0#0000000&|b+0#00e0e07&|a|r| +0#0000000&@63
 @75
-@57|3|0|7|,|1| @8|8|2|%| 
+@57|3|0|7|,|1| @8|7|6|%|