Fix incorrect checkbox placement in Markdown preview

[zfz7]

• Closes Incorrect checkbox placement in Markdown preview. #12515

Before fix:

After fix:

Testing:

• Manual testing
• Added unit test

Test results, these tests fail on the main branch for my setup as well, I have docker running but still had some failures:

failures:
    tests::integration_tests::test_context_collaboration_with_reconnect
    tests::integration_tests::test_formatting_buffer
    tests::integration_tests::test_fs_operations
    tests::integration_tests::test_git_branch_name
    tests::integration_tests::test_git_diff_base_change
    tests::integration_tests::test_git_status_sync
    tests::integration_tests::test_join_after_restart
    tests::integration_tests::test_join_call_after_screen_was_shared
    tests::integration_tests::test_joining_channels_and_calling_multiple_users_simultaneously
    tests::integration_tests::test_leaving_project
    tests::integration_tests::test_leaving_worktree_while_opening_buffer
    tests::integration_tests::test_local_settings
    tests::integration_tests::test_lsp_hover
    tests::integration_tests::test_mute_deafen
    tests::integration_tests::test_open_buffer_while_getting_definition_pointing_to_it
    tests::integration_tests::test_pane_split_left
    tests::integration_tests::test_prettier_formatting_buffer
    tests::integration_tests::test_preview_tabs
    tests::integration_tests::test_project_reconnect
    tests::integration_tests::test_project_search
    tests::integration_tests::test_project_symbols
    tests::integration_tests::test_propagate_saves_and_fs_changes
    tests::integration_tests::test_references
    tests::integration_tests::test_reloading_buffer_manually
    tests::integration_tests::test_right_click_menu_behind_collab_panel
    tests::integration_tests::test_room_location
    tests::integration_tests::test_room_uniqueness
    tests::integration_tests::test_server_restarts
    tests::integration_tests::test_unshare_project
    tests::notification_tests::test_notifications
    tests::random_project_collaboration_tests::test_random_project_collaboration
    tests::remote_editing_collaboration_tests::test_sharing_an_ssh_remote_project

test result: FAILED. 156 passed; 32 failed; 0 ignored; 0 measured; 0 filtered out; finished in 100.98s

Comments:
I do not have a ton of rust knowledge, so very open to feedback. TYSM

Release Notes:

• Fix Incorrect checkbox placement in Markdown preview

[zfz7]

I ran the following commands and they now pass

cargo fmt --all -- --check
cargo clippy --workspace --release --all-targets --all-features -- --deny warnings

[bennetbo]

Thanks for fixing this!
FYI: I changed a bit of the code to make it more "rusty"

[skejeton]

Thank you very much!