[FHL] Add InsertHorizontalLine to AutoFormat functionality

[BryanValverdeU]

Add a new feature to the AutoFormat plugin, whenever in the paragraph there are 3 or more characters of '=', '-' or '_'.
Once pressing enter, the characters should be removed and instead add a HR element:

Horizontal line styles

Following
https://support.microsoft.com/en-us/office/insert-a-horizontal-line-9bf172f6-5908-4791-9bb9-2c952197b1a9

[Copilot]

PR Overview

This pull request adds a new API to insert a horizontal line and integrates auto-formatting functionality to replace paragraphs consisting solely of repeated '-', '=', or '_' characters with a horizontal rule. Key changes include:

• Adding the public API function and its underlying model API implementation for horizontal line insertion.
• Extending unit tests for both public and model APIs as well as auto-format plugin behavior.
• Updating the auto format plugin and demo UI to incorporate the new autoHorizontalLine option.

Reviewed Changes

File	Description
packages/roosterjs-content-model-api/test/publicApi/block/insertHorizontalLineTest.ts	Tests for the public API insertion of horizontal lines.
packages/roosterjs-content-model-api/test/modelApi/block/insertHorizontalLineIntoModelTest.ts	Unit tests verifying model API behavior for horizontal line insertion.
packages/roosterjs-content-model-plugins/test/autoFormat/AutoFormatPluginTest.ts	Extended tests ensuring the auto-format behavior correctly creates a horizontal line for valid inputs.
packages/roosterjs-content-model-api/lib/modelApi/block/insertHorizontalLineIntoModel.ts	New model API function for inserting a horizontal divider.
packages/roosterjs-content-model-api/lib/publicApi/block/insertHorizontalLine.ts	Public API wrapper for the horizontal line insertion functionality.
packages/roosterjs-content-model-plugins/lib/autoFormat/AutoFormatPlugin.ts	AutoFormat plugin updated to detect trigger patterns and insert horizontal lines accordingly.
packages/roosterjs-content-model-plugins/lib/autoFormat/interface/AutoFormatOptions.ts	Extended AutoFormat options with the autoHorizontalLine flag.
demo/scripts/controlsV2/sidePane/editorOptions/Plugins.tsx	Updated demo UI to provide a checkbox for enabling/disabling horizontal line auto-formatting.
packages/roosterjs-content-model-api/lib/index.ts	Export updates to include the new horizontal line APIs.

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (1)

packages/roosterjs-content-model-plugins/lib/autoFormat/AutoFormatPlugin.ts:281

• [nitpick] Consider renaming the arrow function parameter 'c' to 'triggerChar' for improved clarity of intent.

return HorizontalLineTriggerCharacters.some(c => {