Desktop: Fixes #12569: extend auto-pair feature to support fenced code blocks

[bwat47]

Overview

Addresses #12569 by extending the auto-pair feature to support fenced code blocks. When typing three backticks at the start of a line, the editor automatically inserts closing backticks and positions the cursor for quick language specification

Implementation notes

This is a very basic implementation:

• Only triggers the auto-completion when typing 3 backticks on line with no preceding (non whitespace) text
• No list handling (there are many edge cases with code blocks in lists, uses intersectsSyntaxNode to completely avoid triggering auto complete fenced code inside lists)
• No change in behavior for selected text (uses the standard closeBrackets behavior, no attempt to wrap multi-line selected text in code blocks for now)

Testing

• Verify that auto complete of code fence only triggers on line with no preceding text
• Verify that auto complete of code fence doesn't trigger inside lists
• Test normal auto-pair behavior with inline code, quotes, parentheses
• Verify that behavior with selected text is unchanged
• Test with auto-pair disabled and verify behavior is not triggered after typing 3 backticks

Example

Recording.2025-12-05.215357.mp4

[bwat47]

apologies, I'm closing this as I found that it breaks existing plugins that handle creating code blocks (such as better code blocks), if this were added it would probably need to be as a separate setting rather than tied to auto-pair braces (or should be handled by just not adding the 4th backtick per the comment in #12569)