Add support for tables (treetable capability)

[skfriese]

One potential area of improvement would be the addition of support for tabular data, which could be rendered using table rows/cells (treetable). The rows themselves could be expanded to display nested tables, etc. where applicable, or subsequent rows with an indentation added to the first cell, etc.

Our product line is currently utilizing your fantastic vue-tree, and we've had to leverage a separate component to obtain this functionality. It would be useful for us to have this enhancement so we can include just the one component.

A similar approach to this: https://www.primefaces.org/primevue/showcase/#/treetable

[grapoza]

Similar: https://ej2.syncfusion.com/vue/documentation/treegrid/getting-started/

[grapoza]

Moving to 4.0.0 to take advantage of the class-based tree node spec (#189) which in turn was moved to 4.0.0 to avoid having to deal with IE 11.

[grapoza]

With the latest refactorings for 4.0 this has largely become a matter of display and keyboard handling. New Tree.vue and TreeNode.vue parent components can probably handle a large portion of the common props/data/computeds, and inherited TreeView, TreeViewNode, TreeGrid, and TreeGridNode components handle more specific concerns.

Some key things to keep in mind:

• ARIA including focusing and selecting both rows and columns, along with other keyboard navigation
• column headers (and sorting?)
• column alignment

[grapoza]

A partial proof-of-concept is included but hidden in 4.0.3. If all goes well then a final (much more refined and heavily refactored) version will appear in either 5.0.0 or 5.1.0, depending on how intense the 5.0.0 updates are.