Markdown syntax highlighting: line-leading `$` starts a math block and affects the rest of the document

## Description

In a Markdown document, a line beginning with a single dollar sign is interpreted as the start of a multiline math block.

If the line does not contain a closing `$`, the remaining document is highlighted incorrectly. Headings and other Markdown syntax after that line lose their normal highlighting.

This commonly affects Markdown containing shell prompts, commands, or environment variables.

## Steps to reproduce

1. Launch a fresh VS Code window with all extensions disabled:

```sh
code --disable-extensions --new-window vscode-markdown-dollar-repro.md
```

2. Open a Markdown file containing:

```markdown
# Before

$ echo "hello"

# After

**This should be bold**
```

3. Compare the highlighting before and after the line beginning with `$`.

## Actual behaviour

The leading `$` starts a multiline math scope. The remainder of the document is highlighted as math until another dollar sign is encountered, so `# After` and the bold text are no longer highlighted as Markdown.

## Expected behavior

A line-leading `$` that does not form a closed math expression should remain ordinary Markdown text and must not affect tokenisation of subsequent lines.

## Environment

- VS Code: 1.133.0
- Commit: `a5b500951314efd502d07465bd138dfbd714a960`
- Architecture: arm64
- OS: macOS 26.5.1 (25F80)
- Extensions disabled: Yes
- VS Code Insiders: Not tested

## Additional context

This appears related to:

- https://github.com/microsoft/vscode/issues/136535
- https://github.com/microsoft/vscode-markdown-tm-grammar/issues/148

Issue #136535 fixed the case where `$` is followed by a digit, such as `$12.45`. The current single-dollar block rule still treats lines such as `$ command`, `$PATH`, or `$foo` as the beginning of a multiline math block:

https://github.com/microsoft/vscode/blob/main/extensions/markdown-math/syntaxes/md-math-block.tmLanguage.json#L48-L50

The relevant rule is:

```regex
(?<=^\s*)(\$)(?![^$]*\$|\d)
```