separate import, elk layout and init-js into different sub category

[Eymeric65]

I do think it need some quick discussion about how to treat the init-js behavior. As stated in #172, documentation may lake some information about the behavior of init-js. I do think that init-js config variable shouldn't be linked with mermaid version and elk import and following this I propose this code modification to separate these three behavior.

[Eymeric65]

I also found really strange behavior when using sphinx config variable as a 'running' variable like this :

    if app.config.mermaid_init_js == _MERMAID_INIT_JS_DEFAULT:
        # Update if esm is used and no custom init-js is provided
        if _mermaid_elk_js_url:
            # Add registration of ELK layouts
            app.config.mermaid_init_js = f'import mermaid from "{_mermaid_js_url}";import elkLayouts from "{_mermaid_elk_js_url}";mermaid.registerLayoutLoaders(elkLayouts);{app.config.mermaid_init_js}'
        else:
            app.config.mermaid_init_js = f'import mermaid from "{_mermaid_js_url}";{app.config.mermaid_init_js}'

    if app.config.mermaid_init_js:
        # If mermaid is local the init-call must be placed after `html_js_files` which has a priority of 800.
        priority = app.config.mermaid_init_js_priority if _mermaid_js_url is not None else 801
        app.add_js_file(None, body=app.config.mermaid_init_js, priority=priority, type="module")

it may be better to adopt a safer approach like this one :

    mermaid_init_js = app.config.mermaid_init_js

    if _mermaid_elk_js_url:
        # Add registration of ELK layouts
        mermaid_init_js = f'import elkLayouts from "{_mermaid_elk_js_url}";mermaid.registerLayoutLoaders(elkLayouts);{mermaid_init_js}'


    if mermaid_init_js:

        # Add the mermaid import from link to the init-js
        mermaid_init_js = f'import mermaid from "{_mermaid_js_url}";{mermaid_init_js}'
        
        # If mermaid is local the init-call must be placed after `html_js_files` which has a priority of 800.
        priority = app.config.mermaid_init_js_priority if _mermaid_js_url is not None else 801
        app.add_js_file(None, body=mermaid_init_js, priority=priority, type="module")