Fix heredoc leading whitespace stripping

[tobiaslins]

I found this when trying to store and read a ASCII triangle using just-bash

[vercel] 🔧 Tool call: bash
[vercel] 📝 Command: cat > triangle.txt << 'EOF'
                    *
                   * *
                  *   *
                 *     *
                *       *
               *         *
              *           *
             *             *
            *               *
           *                 *
          *                   *
         *********************
EOF
cat triangle.txt

[vercel] ✅ Exit code: 0
[vercel] stdout: *
* *
*   *
*     *
*       *
*         *
*           *
*             *
*               *
*                 *
*                   *
*********************

[vercel] 🏁 Completed - 1 tool calls, response length: 65

The heredoc content was losing leading whitespace due to two issues:

1. In lexer.ts, skipWhitespace() was called before checking for pending heredocs, which stripped leading whitespace from the first line of heredoc content. Fixed by moving the heredoc check before skipWhitespace().

2. In Bash.ts, the script normalization (trimStart on every line) was stripping whitespace from heredoc content. Fixed by making the normalizeScript function heredoc-aware so it preserves content inside heredocs while still normalizing the surrounding script.

Added tests for whitespace preservation in heredocs.

[vercel]

@tobiaslins is attempting to deploy a commit to the Vercel Labs Team on Vercel.

A member of the Team first needs to authorize it.