feat: allow customizing icons via icons prop

[sleitor]

Summary

Two features in one PR:

1. Custom icons — override any built-in icon (copy, download, zoom, etc.) via a new icons prop on <Streamdown>
2. Custom starting line numbers — specify a starting line number in code fences via startLine=N meta string

Custom icons

New icons prop accepts a Partial<IconMap> — unspecified icons fall back to defaults.

import { Streamdown, type IconMap } from "streamdown";

const MyCheckIcon = (props) => <svg {...props}>...</svg>;

<Streamdown icons={{ CheckIcon: MyCheckIcon }}>
  {content}
</Streamdown>

Changes

• New lib/icon-context.tsx with IconMap type, IconProvider, and useIcons() hook
• All sub-components updated to consume icons via useIcons() instead of direct imports
• IconMap type exported from package entry
• Follows the existing components prop pattern using React context

Custom starting line numbers

Code blocks can specify a starting line number in the meta string:

```js startLine=10
const x = 1;
```

Changes

• New remarkCodeMeta remark plugin that forwards fenced-code meta strings to hast
• metastring attribute allowed through rehype-sanitize
• startLine=N parsed in CodeComponent and passed down as a prop
• CodeBlockBody applies counter-reset: line N-1 as an inline style
• Includes bounds checking (values < 1 are ignored)

Closes #412, closes #287

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[haydenbleasel]

thank you @sleitor 🙏