Add flowchart graph format

Here's a first pass at implementing the Mermaid Flowchart format.

Note that I also overrode the template to use a <pre> tags, since that appears to be the suggested approach now. This also has the nice benefit of making the Views preview more legible. I can split this into a separate issue, if that's preferable.

This currently hard-codes vertices rendered as circles, and that the flowchart will be left-to-right. It also doesn't implement the 'click' behaviour. I think I'll need to extend GraphAPI's Views style plugin (and use hook_views_plugins_style_alter()) to address that stuff. But I'm certainly open to other suggestions.

I've made a bunch more progress on this, but let's get 📌 Move GraphAPI support to a submodule Needs review done first, so that this feature can more cleanly be added to the GraphAPI sub-module.

I'm going to move to a git-based workflow, so that 📌 Move GraphAPI support to a submodule Needs review can be a separate branch, that we then base the work here (and in ✨ Add flowchart graph format Fixed ) off of.

So as not to inadvertently lost any of the work I've done so far, I'll just attach a patch of my latest iteration on this.

I updated the issue summary with my progress so far, and some planned next steps.

It will probably be easier to define our own Views style plugin, rather than taking over and altering the default GraphAPI one. We'd still inherit from that one, but I don't like the idea of wrapping a bunch of conditional logic around the Mermaid-specific code.

I've implemented a new views style plugin just for Mermaid graphs. I also overrode the default GraphAPI views style plugin, to remove the Mermaid graphs. I'm not really sure this is the best approach, but I couldn't figure out anything better.

Flowchart edge styles are now configurable.

Flowchart direction is now configurable.

Linking vertices their entities is now optional.

Edge labels are now supported.

Merged to 1.0.x.

I tested that providing our own views style plugin doesn't break the existing erDiagram graph plugin. It does add some options in the views settings that aren't currently supported by that plugin (eg. dynamic vertex shape and edge styling). But I think we can deal with that in a separate issue. The simplest solution would be to have the erDiagram plugin go back to using the default GraphAPI views style plugin.

Automatically closed - issue fixed for 2 weeks with no activity.