markdown: Don't adjust indentation when inserting with multiple cursors (#40794)

Closes #40757

## Summary

This PR addresses an issue where Zed incorrectly adjusts the indentation
of Markdown lists when inserting text using multiple cursors. Currently:

- Editing individual lines with a single cursor behaves correctly (no
unwanted indentation changes).
- Using multiple cursors, Zed automatically adjusts the indentation,
unlike VS Code, which preserves the existing formatting.

## Tasks
- [x] Implement a new test to verify correct Markdown indentation
behavior with multiple cursors.
- [x] Apply the fix to prevent Zed from adjusting indentation when
inserting text on multiple cursors.

------------------------

Release Notes:

- Fixed an issue where inserting text with multiple cursors inside a
nested Markdown list would cause it to lose its indentation.

---------

Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>

crates/languages/src/markdown/config.toml

@@ -24,4 +24,5 @@ rewrap_prefixes = [
 auto_indent_on_paste = false
 auto_indent_using_last_non_empty_line = false
 tab_size = 2
+decrease_indent_pattern = "^\\s*$"
 prettier_parser_name = "markdown"

crates/languages/src/markdown/indents.scm

@@ -0,0 +1,3 @@
+(list (list_item) @indent)
+
+(list_item (list) @indent)