feat: add streamdown/core entry without rehype-raw

[sleitor]

Description

Adds a streamdown/core entry whose default rehype plugins omit rehype-raw, so rehype-raw and parse5 are not bundled. The default streamdown entry is unchanged (raw HTML stays on).

Type of Change

• New feature (non-breaking change which adds functionality)

Related Issues

Closes #660

Changes Made

• lib/markdown.ts no longer imports rehype-raw. Detection goes through a small registry (lib/raw-registry.ts), with a plugin.name === "rehypeRaw" fallback for user-supplied plugins. remarkEscapeHtml behaviour is unchanged and the processor cache key is untouched.
• lib/default-raw.ts is now the only module importing rehype-raw; it registers the plugin. index.tsx uses it for defaultRehypePlugins.raw and the allowedTags path.
• tsup.config.ts builds a second entry, core, with an esbuild plugin that swaps default-raw for a stub (lib/default-raw.core.ts).
• package.json: new ./core export. Docs: note in configuration page.

Bundle size (esbuild --bundle --minify, react/shiki external, importing Streamdown):

entry	size	parse5
streamdown	530,705 B	yes
streamdown/core	336,317 B	no

Caveat: with streamdown/core, a user-supplied rehype-raw is detected by function name (rehypeRaw), which a minifier could rename; in that case HTML would be escaped.

Testing

• All existing tests pass
• Added new tests for the changes

New __tests__/raw-html-detection.test.tsx: default renders raw HTML; custom plugins without raw still escape; user-supplied rehypeRaw (with and without options) is detected.

Checklist

• My code follows the project's code style
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have created a changeset (pnpm changeset)

Changeset

• I have created a changeset for these changes

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