Missing ARIA on nimble-table

[fredvisser]

🐛 Bug Report

Per the ARIA Treegrid example and the ARIA APG Grid and Table Properties documents, we are missing a few ARIA properties required to fully describe the nimble-table for screenreaders.

Missing label	Role	Description
aria-label="Inbox"	treegrid	Provides an accessible name for the treegrid.
aria-level="number"	row	Defines the level of the row in the hierarchical treegrid structure. Counting is one-based. Root rows have aria-level=“1”.
aria-setsize="number"	row	Defines the number of rows in the set of rows that are in the same branch and at the same level within the hierarchy.
aria-posinset="number"	row	Defines the position of the row within the set of other rows that are in the same branch and at the same level within the hierarchy. Counting is one-based, not zero-based.
Change role="cell" to role="gridcell"	cell	Identifies the element as a gridcell. The gridcell role is not an implicit semantic for the tdelement when in a treegrid.

Property	Definition
aria-colcount	Defines the total number of columns in a table, grid, or treegrid.
aria-rowcount	Defines the total number of rows in a table, grid, or treegrid.
aria-colindex	Defines a cell's position with respect to the total number of columns within a table, grid, or treegrid. Note: Numbering starts with 1, not 0.
aria-rowindex	Defines a cell's position with respect to the total number of rows within a table, grid, or treegrid. Note: Numbering starts with 1, not 0.

There are also some accessibility errors on the table in the Lighthouse report. We should attempt to resolve these if they remain after applying the above ARIA.

Some ARIA parent roles must contain specific child roles to perform their intended accessibility functions. [Learn more about roles and required children elements](https://dequeuniversity.com/rules/axe/4.9/aria-required-children).
LINKDATESTATUSNUMBERDURATIONSTRING 2COLOR
<nimble-table _ngcontent-ng-c1457765990="" id-field-name="id" parent-id-field-name="parentId" selection-mode="multiple" role="treegrid" tabindex="0" aria-multiselectable="true">
div.table-container > div.glass-overlay > div.table-viewport
<div class="table-viewport" tabindex="-1">
FIRST NAMELAST NAMEAGE
<nimble-table _ngcontent-ng-c1457765990="" id-field-name="id" parent-id-field-name="parentId" selection-mode="multiple" role="treegrid" tabindex="0" aria-multiselectable="true">
div.table-container > div.glass-overlay > div.table-viewport
<div class="table-viewport" tabindex="-1">