Add render --page-break-before to start chapters on new pages

[odrobnik]

What

Adds an optional --page-break-before <h1–h6> option to swifttext render. When set, it forces a page break before every heading of that level in PDF/HTML output — e.g. --page-break-before h2 starts each chapter on its own page.

Also adds Scripts/manuscript-to-pdf.sh, which builds the CLI and renders a Markdown manuscript to PDF with --page-break-before h2 end to end.

Why

The render command's print CSS had no forced page breaks for headings, and the render path escapes raw HTML in the Markdown (passThroughRawHTML is off). So a caller had no way to make each chapter begin on a fresh page — neither via CSS nor by injecting markup. This closes that gap with a small, explicit option.

How

• markdownToHTML gains a defaulted pageBreakBefore: HeadingBreakLevel? parameter and injects break-before: page / page-break-before: always for the chosen level, placed after the base heading rules so it wins on source order. The leading break is ignored by the print engine when the heading is the first box, so no blank first page is emitted.
• New HeadingBreakLevel enum (h1–h6, ExpressibleByArgument) and --page-break-before option on Render, threaded through the .html and .pdf cases.
• The default is off, and the PDF subcommand's markdownToHTML callers are untouched (they rely on the defaulted parameter), so existing output is unchanged.

Notes for reviewers

• Scope is PDF/HTML only; DOCX and Pages go through separate converters and are unaffected (reflected in the option's help text).
• Verified against a 16-chapter manuscript: each ## heading lands at the top of its own page, with no page carrying two headings.
• SwiftLint is clean on both changed files; the swifttext product builds and runs.

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