editor: Fix auto-indent cases in Markdown (#44616)

Builds on https://github.com/zed-industries/zed/pull/40794 and
https://github.com/zed-industries/zed/pull/44381

- Fixes the case where creating a new line inside a nested list puts the
cursor correctly under that nested list item.
- Fixes the case where typing a new list item at the expected indent no
longer auto-indents or outdents incorrectly.

Release Notes:

- Fixed an issue in Markdown where new list items weren’t respecting the
expected indentation on type.

crates/languages/src/markdown/config.toml

@@ -24,5 +24,9 @@ rewrap_prefixes = [
 auto_indent_on_paste = false
 auto_indent_using_last_non_empty_line = false
 tab_size = 2
-decrease_indent_pattern = "^.*$"
+decrease_indent_patterns = [
+  { pattern = "^\\s*-",    valid_after = ["list_item"] },
+  { pattern = "^\\s*\\d",    valid_after = ["list_item"] },
+  { pattern = "^\\s*",    valid_after = ["list_item"] },
+]
 prettier_parser_name = "markdown"

crates/languages/src/markdown/indents.scm

@@ -1 +1,3 @@
 (list (list_item) @indent)
+
+(list_item) @start.list_item