Make the code macro editable in-place in BlockNote

Currently the BlockNote editor displays the code macro calls like any other macro call: the macro output is read-only and the code can be modified only through the macro edit dialog.

The code macro calls should be edited in-place, as BlockNote code blocks, with syntax highlighting and a language selector:

• the language parameter is mapped to the language of the code block, and the other macro parameters (e.g. layout) are preserved on save;
• new code blocks are saved as code macro calls, while the code blocks that come from verbatim blocks ({{{...}}}) are still saved as verbatim blocks;
• code macro calls that are inline or that take the code from a different source (the source parameter) remain read-only macro calls;
• the code blocks look the same in edit mode as the code macro output in view mode.