runtime(vim): Update base-syntax, improve variable matching

- Match variables after operators, including line continuations.
- Match option variables without leading whitespace.
- Explicitly match expression subscripts.
- Match Vim9 variables in LHS of assignments and method calls.
- Match option variables (&option) with a dedicated syntax group like
  environment variables.
- Match list literals, fixes: #5830
- Match :{un}lockvar arguments.
- Match registers and environment variables in :let unpack lists.
- Match lambda expressions
- Match Vim9 scope blocks
- Match variables in :for subject
- Highlight user variables with Normal
- Improve this/super keyword matching, fixes: #15970

closes: #16476

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

runtime/syntax/testdir/dumps/vim_ex_def_nested_01.dump

@@ -1,9 +1,10 @@
-| +0&#ffffff0@7|e+0#af5f00255&|n|d@1|e|f| +0#0000000&@60
+| +0&#ffffff0@11|r+0#af5f00255&|e|t|u|r|n| +0#0000000&|G|i|v|e|N|a|m|e|(+0#e000e06&|)| +0#0000000&@45
+@8|e+0#af5f00255&|n|d@1|e|f| +0#0000000&@60
 @75
-@8|t+0#00e0e07&|h|i|s|.+0#af5f00255&|n+0#0000000&|a|m|e| |=+0#af5f00255&| +0#0000000&|N|a|m|e|(+0#e000e06&|)| +0#0000000&@48
+| +0#00e0e07&@7|t+0#0000001#ffff4012|h|i|s|.+0#af5f00255#ffffff0|n+0#00e0e07&|a|m|e| +0#0000000&|=+0#af5f00255&| +0#0000000&|N|a|m|e|(+0#e000e06&|)| +0#0000000&@48
 @4|e+0#af5f00255&|n|d@1|e|f| +0#0000000&@64
-|e+0#af5f00255&|n|d|c|l|a|s@1| +0#0000000&@66
-> @74
+>e+0#af5f00255&|n|d|c|l|a|s@1| +0#0000000&@66
+@75
 |e+0#af5f00255&|c|h|o| +0#0000000&|T|e|s|t|.|n+0#00e0e07&|e|w|(+0#e000e06&|)| +0#0000000&@59
 |~+0#4040ff13&| @73
 |~| @73
@@ -16,5 +17,4 @@
 |~| @73
 |~| @73
 |~| @73
-|~| @73
-| +0#0000000&@56|1|9|,|0|-|1| @7|B|o|t| 
+| +0#0000000&@56|1|9|,|1| @9|B|o|t|