Add Vim9Script support

Parse core Vim9Script constructs into proper AST nodes so that
downstream tools can analyze Vim9Script code without errors.

New node types: NODE_VIM9SCRIPT, NODE_DEF, NODE_ENDDEF, NODE_VAR,
NODE_FINAL, NODE_EXPORT, NODE_IMPORT.

Parsers for vim9script, def/enddef (with typed parameters, return
types, default values, variadics), var/final (with type annotations),
export (wrapping inner commands), and import.

The # comment syntax is supported when vim9script mode is active.

Also fix JS transpiler heuristic for subscript assignments in
compile_let to check for [ in addition to . when deciding whether
to add var prefix.

Fixes #194

Author: sideshowbarker

autoload/vimlparser.vim

@@ -352,7 +352,7 @@ function s:JavascriptCompiler.compile_let(node)
       call self.out('%s.prototype.%s %s %s;', klass, name, op, right)
       return
     endif
-    if left =~ '\.' || op != '='
+    if left =~ '[\.\[]' || op != '='
       call self.out('%s %s %s;', left, op, right)
     else
       call self.out('var %s %s %s;', left, op, right)