fix(block): constrain line-number gutter width on the frontend

[faisalahammad]

Fixes #287

Changes proposed in this Pull Request

• The Gutenberg block output (.wp-block-syntaxhighlighter-code) uses the v3 highlighter table, where table-layout: auto + width: 100% on the table and td.code stretches the td.gutter cell, so line numbers sit far from the code. On narrow screens the gutter and code cells also split onto separate rows.
• Adds two block-scoped CSS rules in src/code-block/style.scss (built into dist/blocks.style.build.css): max-width: fit-content on the gutter cell, and display: inline-table on the row so the number stays on the same line as its code.
• The file dist/blocks.style.build.css was built but never loaded before. It is now registered as syntaxhighlighter-blocks-css and injected on the frontend only when the page contains a syntaxhighlighter/code block (or a reusable block), using the same JS link-injection pattern maybe_output_scripts() already uses for the core/theme styles.
• Shortcode output ([sourcecode] etc.) has no block wrapper, so it is untouched.

Testing instructions

• On a site running Twenty Twenty-Four, add a SyntaxHighlighter Code block, paste some code, and turn "Line numbers" on. Publish.
• On the frontend, the line-number column hugs the numbers instead of stretching toward the center.
• Resize the window narrow (under ~640px). Each line number stays on the same row as its code line.
• Add a classic [sourcecode php] shortcode on another post. Its gutter/layout looks unchanged.
• Tested manually on WordPress 6.7.2, Twenty Twenty-Four, Chrome/Firefox/Safari, with the plugin built from this branch (npm run build).