Improve 3-backtick to insert code blocks

[cyanzhong]

No description provided.

[Copilot]

Pull request overview

This PR improves Markdown fenced code block insertion behavior in the CoreEditor, especially when typing backticks, by switching to snippet-based insertion with placeholders and by allowing snippets to target an explicit replacement range.

Changes:

• Extend insertSnippet() to optionally replace an explicit { from, to } range rather than always using the main selection.
• Rework backtick input handling to convert `` into a fenced code block with placeholders and ensure fences are placed on their own lines, avoiding insertion while already inside code.
• Update the “Insert Code Block” command to use snippet placeholders for language/content when selection is a single, single-line range.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
CoreEditor/src/modules/snippets/insertSnippet.ts	Adds an optional replacement range to support non-selection snippet insertions.
CoreEditor/src/modules/input/insertCodeBlock.ts	Replaces prior dispatch-based insertion with snippet-based fenced block insertion + syntax-tree checks.
CoreEditor/src/modules/commands/index.ts	Updates the code block command to use snippet placeholders when safe (single selection, single line).

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.