This PR fixes the handling of some border CSS properties like border-color that incorrectly ended up being translated to border specifications when they shouldn't. Its also updates the tests to check this.

The test file adds a test for setting border-color, which was the issue reported. It also moves the background check out of the border section and into a new border test section.

The ts/util/Styles.ts file is modified as follows:

The connection data adds a new parts array to list the sub-properties like border-color that modify other styles like border-top-color but that don't combine directly to the border property. It also adds a subPart boolean to indicate a property like border-color that acts in that way. Previously, we used combine: null to indicate this, but that was not a very clear indication.

A new combinePart() function is used for border-color and the other ones like it to handle the adjustments to the styles for those. This uses combineTRBL() to first change the properties like border-top-color, then calls combineChildren() to see if a combined border property can now be used, then calls combineSame() to see if border-color can be used in place of border-top-color, etc., and finally calls the new combineParent() function described below.

The combineWSC() function is modified to only combine only if there is more than one of the three properties that is specified, so border-color: red alone won't move to border: red.

The connection definitions are modified to mark border as having width, style, and color sub-parts, and the definitions for those now use combine: combinePart and subPart: true.

The cssText output now checks whether a combined part property is available before including the more specific ones, so that border-top-color will not be included if border-color is available.

The set() function is refactored to move the code that handles combining changes into parent properties into a separate combineParent() function (since it is now needed in several places). It also now uses the subPart connection property rather than combine === null to test for border-color and similar properties, and in then calls the connection's combine() function, which now points to combinePart() for these border properties. Finally, after combining parents, if the property is a specific sub-part (like border-top-color), now the it is changed, we check to see if the other TRBL parts can be combined (e.g., as border-color).

The code for combineParent() is taken from the old set() definition, and simplified a bit (caching the Styles.connect[name] locally as connect), and then adds code to remove sub-parts when the child properties have been combined (so if border is able to be used, the border-color and other sub-parts will not be needed).

The rest of the changes are simplifications or comment clean-ups.

Resolves issue mathjax/MathJax#3490.