Add Vim9Script support

[sideshowbarker]

Summary

• Parse core Vim9Script constructs into proper AST nodes instead of routing them through parse_cmd_common, so downstream tools can analyze Vim9Script code without errors
• Add # comment support when vim9script mode is active
• Fix JS transpiler compile_let heuristic that incorrectly added var prefix to subscript assignments like named[pname] = 1

Motivation

Tools that depend on vim-vimlparser — such as vint and vim-language-server (which vendors the JS output directly) — report unexpected errors on valid Vim9Script code because the parser doesn't recognize constructs like def, typed parameters, or # comments. This adds structured parsing for the core Vim9Script syntax so these tools can handle Vim9Script files correctly.

Details

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

• vim9script — sets a mode flag that enables # comments throughout the parser (in parse_one_cmd, parse_command, parse_comment, parse_trail, ends_excmds, separate_nextcmd).

• def/enddef — parses typed parameters (name: type), default values (name: type = expr), variadic params (...name: type), and return type annotations (: returntype). Reads parameter names directly from the reader rather than using ExprTokenizer, since : is a valid name character in VimL identifiers and would cause x: to be tokenized as a single identifier.

• var/final — parses variable declarations with optional type annotations. Same direct-reader approach to avoid the : tokenization issue.

• export — wraps the inner command (def, var, etc.) by delegating to find_command + _parse_command, then moving the resulting nodes into the export node's body.

• import — captures the import specification as a string.

Fixes #194

[sideshowbarker]

I started on this before seeing #204. The approach in that PR is clearly a much better solution than this. (I hadn’t even known yet about https://github.com/vim-jp/vim-vim9parser.) But I figured I’d go ahead and raise this anyway — in case it happens to be of any interest, or might be a possible interim solution until #204 gets merged.