Styling the edge lines with line-outline properties (border)

[xkdgo]

Description of new feature

In addition to existing overlays/underlays add new properties to edges
line-outline-width and line-outline-color

Those properties are add new border visualisation of edge when line-outline-width > 0

Motivation for new feature

This feature is usable when use library for network weathermap. There was a dialogue about this feature.
When there is zero traffic between nodes it can be shown in white, but this is impossible now when the background is also white.
Underlay can resolve this problem only when line is not dashed.
When the feature is implemented , overlay/underlay can be used when highlighting a line.
In example below

      "style": {
          // existed properties
          "width": 3,
          "line-color": "white",
          "underlay-opacity": "0.3",
          "underlay-color": "red",
          "underlay-padding": "3px",
          "line-style": "dashed",
          "line-dash-pattern": [15, 15],
          
          // new feature proposal
          "line-outline-width": 2, // default 0 mean switched off
          "line-outline-color": "#000" // default "#000"
      }

visualisation will be

For reviewers

Reviewers should ensure that the following tasks are carried out for incorporated issues:

• Ensure that the reporter has adequately described their idea. If not, elicit more information about the use case. You should iteratively build a spec together.
• Ensure that the issue is a good fit for the core library. Some things are best done in extensions (e.g. UI-related features that aren't style-related). Some things are best done by app authors themselves -- instead of in Cytoscape libraries.
• The issue has been associated with a corresponding milestone.
• The commits have been incorporated into the unstable branch via pull request. The corresponding pull request is cross-referenced.