Add on-type formatting with cancellation token support

[Copilot]

Description

Implements OnTypeFormattingEditProvider with ; and } triggers. Enhances all formatting providers with cancellation token support.

On-Type Formatting

• Triggers on ; (semicolon) and } (closing brace)
• Validates trigger character at exact cursor position
• Only formats when trigger is at end of line (prevents mid-edit formatting)
• Controlled via standard VS Code editor.formatOnType setting

Cancellation Token Support

• Added to PrettierEditProvider (document/range formatting)
• Added to PrettierOnTypeFormattingEditProvider
• Added to PrettierCodeActionProvider
• Checks cancellation before and after format operations

Files

• src/PrettierOnTypeFormattingEditProvider.ts - New provider implementation
• src/PrettierEditService.ts - Provider registration and cancellation plumbing
• src/PrettierEditProvider.ts - Cancellation support
• src/PrettierCodeActionProvider.ts - Cancellation support
• src/test/suite/onTypeFormatting.test.ts - Test coverage
• docs/on-type-formatting.md - User documentation

Related Issue

Fixes #3652

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactoring (no functional changes)
• CI/Build changes

Checklist

• I have read the CONTRIBUTING guidelines
• My code follows the code style of this project (npm run lint passes)
• I have run npm run prettier to format my code
• I have added/updated tests that prove my fix or feature works
• All new and existing tests pass (npm test)
• I have updated the CHANGELOG.md with a summary of my changes

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add on type formatting</issue_title>
<issue_description>Implement on type code formatting (https://code.visualstudio.com/api/references/vscode-api#OnTypeFormattingEditProvider) and review the implementation of the range edit provider for any improvements that could be made.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add on type formatting #3869

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