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/input/vim_expr.vim

@@ -85,6 +85,21 @@ echo 0zFF00ED015DAF
 echo 0zFF00.ED01.5DAF
 echo 0zFF.00.ED.01.5D.AF
 
+" List
+
+echo []
+echo [42]
+echo [[11, 12], [21, 22], [31, 32]]
+echo [1,
+      \ 2,
+      \ 3,
+      \ 4
+      \]
+echo [1, 'two', 1 + 2, "fo" .. "ur"]
+
+" Issue #5830 (Incorrect syntax highlighting in Vim script when omitting space in list of string)
+let l = ['a','b','c']
+
 " Operators
 
 " Ternary
@@ -169,9 +184,26 @@ echo expr !~?    expr
 echo expr is?    expr
 echo expr isnot? expr
 
-" Unreported issue (incorrectly matches as "echo vimNumber *vimCommand* vimNumber")
+" Unreported issue ("is" incorrectly matches as "echo vimNumber *vimCommand* vimNumber")
 echo 42 is 42
 
+" Line continuation
+let foo = foo +
+      \
+      "\ comment
+      \
+      "\ comment
+      \ bar +
+      \ "baz"
+
+let foo = foo +
+      "\ comment
+      \
+      "\ comment
+      \
+      \ bar +
+      \ "baz"
+
 
 " Issue #16221 (vimString becomes vimVar when preceded by !)
 let bar = !'g:bar'->exists()