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/vim9_def_variables.vim

@@ -0,0 +1,200 @@
+vim9script
+
+# Vim9 function variable highlighting
+
+def Foo()
+  # Declarations
+
+  var foo = expr
+
+  b:foo = expr
+  g:foo = expr
+  t:foo = expr
+  w:foo = expr
+
+  v:foo = expr
+
+  $FOO = expr
+
+  var [foo, bar] = expr
+  var [foo,
+	\ bar] = expr
+  var [$foo, $bar] = expr
+  var [$foo,
+	\ $bar] = expr
+
+  var [foo, bar; baz] = expr
+  var [foo,
+	\ bar;
+	\ baz] = expr
+  var [$foo, $bar; $baz] = expr
+  var [$foo,
+	\ $bar;
+	\ $baz] = expr
+
+  var foo =<< END
+...
+END
+  var foo =<< trim END
+  ...
+  END
+  var foo =<< eval END
+...
+END
+  var foo =<< trim eval END
+  ...
+  END
+  var foo =<< eval trim END
+    ...
+  END
+
+  # Assignments
+
+  foo = expr
+
+  foo[0] = expr
+
+  foo[1:2] = expr
+  foo[:2] = expr
+  foo[1:] = expr
+  foo[:] = expr
+
+  foo += expr
+  foo -= expr
+  foo *= expr
+  foo /= expr
+  foo %= expr
+  foo ..= expr
+
+  b:foo = expr
+  g:foo = expr
+  t:foo = expr
+  w:foo = expr
+
+  b:foo += expr
+  g:foo += expr
+  t:foo += expr
+  w:foo += expr
+
+  b:foo -= expr
+  g:foo -= expr
+  t:foo -= expr
+  w:foo -= expr
+
+  b:foo *= expr
+  g:foo *= expr
+  t:foo *= expr
+  w:foo *= expr
+
+  b:foo /= expr
+  g:foo /= expr
+  t:foo /= expr
+  w:foo /= expr
+
+  b:foo %= expr
+  g:foo %= expr
+  t:foo %= expr
+  w:foo %= expr
+
+  b:foo ..= expr
+  g:foo ..= expr
+  t:foo ..= expr
+  w:foo ..= expr
+
+  $FOO = expr
+  $FOO ..= expr
+
+  @f = expr
+  @f ..= expr
+
+  &foo = expr
+
+  &t_k1 = "\<Esc>[234;"
+
+  &foo ..= expr
+
+  &foo += expr
+  &foo -= expr
+
+  &l:foo = expr
+
+  &l:foo ..= expr
+  &l:foo += expr
+  &l:foo -= expr
+
+  &g:foo = expr
+
+  &g:foo ..= expr
+  &g:foo += expr
+  &g:foo -= expr
+
+  [foo, bar] = expr
+  [foo,
+	\ bar] = expr
+  [v:foo, v:bar] = expr
+  [v:foo,
+	\ v:bar] = expr
+  [&foo, &bar] = expr
+  [&foo,
+	\ &bar] = expr
+  [$foo, $bar] = expr
+  [$foo,
+	\  $bar] = expr
+  [@a, @b] = expr
+  [@a,
+	\  @a] = expr
+
+  [foo, bar] ..= expr
+  [foo, bar] += expr
+  [foo, bar] -= expr
+  [foo, bar] *= expr
+  [foo, bar] /= expr
+  [foo, bar] %= expr
+
+  [foo, bar; baz] = expr
+  [foo,
+	\ bar;
+	\ baz] = expr
+  [v:foo, v:bar; v:baz] = expr
+  [v:foo,
+	\ v:bar;
+	\ v:baz] = expr
+  [$foo, $bar; $baz] = expr
+  [$foo,
+	\ $bar;
+	\ $baz] = expr
+  [&foo, &bar; &baz] = expr
+  [&foo,
+	\ &bar;
+	\ &baz] = expr
+  [@a, @b; @c] = expr
+  [@a,
+	\ @b;
+	\ @c] = expr
+
+  foo =<< END
+...
+END
+  foo =<< trim END
+  ...
+  END
+  foo =<< eval END
+...
+END
+  foo =<< trim eval END
+  ...
+  END
+  foo =<< eval trim END
+    ...
+  END
+
+  # :for
+
+  for foo in expr
+  endfor
+
+  for [foo, bar] in expr
+  endfor
+enddef
+
+