* feat(nested collections): allow non-index files

This commit fixes #4972 to allow nested folders with additional content
beyond an index file.

Side effect: To keep the feature simple, this will now show index files
as pages within a folder in NetlifyCMS. This enables creating additional
files alongside the given index, but is a change in behavior from the
current implementation.

Co-authored-by: Eric Gade <105373963+eric-gade@users.noreply.github.com>

* test(e2e): adapt to new way nested collections work

 We use regexps as otherwise .contains("Sub Directory") would also match "Another Sub Directory"

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Co-authored-by: Andrew Dunkman <adunkman@gmail.com>
Co-authored-by: Eric Gade <105373963+eric-gade@users.noreply.github.com>
Co-authored-by: Anze Demsar <anze.demsar@p-m.si>

Extends the HTML to Markdown conversion to better support bold and
italic formatting from Google Docs, which generates inline styles on a
`span` element instead of strong/b/em/i type elements.

Co-authored-by: Anze Demsar <anze.demsar@p-m.si>

* refactor: clean up controlRef handling

* feat: add click-to-edit

* test: update snapshots

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Co-authored-by: Anze Demsar <anze.demsar@p-m.si>

When using a Variable Type list widget and a custom preview component,
the `widgetsFor` helper would only return a `data` list with each of the
items in the list, not a `widgets` list, e.g.

    {"data" =>
        {"markdown" => "# Title"}
        {"type" => "block_body"}
    }
    {"widgets" => undefined} 🚫

The `widgets` list should also be supplied, particularly for nested
Markdown widgets, so a fully formatted preview can be rendered:

    {"data" =>
        {"markdown" => "# Title"}
        {"type" => "block_body"}
    }
    {"widgets" =>
        {"markdown" => Object} ✅
    }

This extends support in `widgetsFor` to detect variable type list
widgets and correctly construct the `widgets` return value.

As reported at #2307 (comment)

Co-authored-by: Anze Demsar <anze.demsar@p-m.si>

 - decap-cms@3.6.0
 - decap-cms-app@3.6.0
 - decap-cms-core@3.6.0
 - decap-cms-lib-util@3.2.0
 - decap-cms-widget-list@3.3.0
 - decap-cms-widget-markdown@3.3.0
 - decap-cms-widget-object@3.3.0
 - decap-server@3.2.0