feat: export InsertionMode enum from parse5

[TrevorBurnham]

Closes #531

Summary

Exports the InsertionMode enum so that consumers building custom parsers can check insertionMode against named values instead of magic numbers.

Changes

• packages/parse5/lib/parser/index.ts — Changed enum InsertionMode to export enum InsertionMode with explicit numeric assignments to guarantee stable values across releases.
• packages/parse5/lib/index.ts — Added InsertionMode to the re-export from ./parser/index.js.
• packages/parse5/lib/parser/index.test.ts — Added a smoke test verifying the export exists and key values are stable.

Usage

import { InsertionMode } from 'parse5';

// Now you can reference InsertionMode.IN_BODY, InsertionMode.IN_TABLE, etc.
// instead of comparing against raw numeric values.