fix: remove the @internal decorator for the method getPlainText of document

[HerbertHe]

Description

The toPlainText() method of document is not work correctly for the custom embed builders, then I have read the source code. And I found that the alternative way is to use getPlainText method and pass the whole document range as params to meet the same effect as toPlainText method.

But, I got the warning when I set the last param includeEmbeds's value as true. So, the PR is to fix the warning caused by the decorator @internal.

Related Issues

Type of Change

• ✨ Feature: New functionality without breaking existing features.
• 🛠️ Bug fix: Resolves an issue without altering current behavior.
• 🧹 Refactor: Code reorganization, no behavior change.
• ❌ Breaking: Alters existing functionality and requires updates.
• 🧪 Tests: New or modified tests
• 📝 Documentation: Updates or additions to documentation.
• 🗑️ Chore: Routine tasks, or maintenance.
• ✅ Build configuration change: Build/configuration changes.