"Reverse Viewport" support in the JS API

[niloc132]

JS API component of #1946.

JsTable.setViewport will change its api to support an object full of options:

• start/end (possibly also rangeset)
• columns (optional, defaults to all)
• updateinterval (optional, defaults to "server picks" or 1000ms)
• reverse (optional, defaults to false)

The old API will still be supported as well for a release or two, but will not support the new reverse flag. This API change, possibly without the new reverse flag (at least not right away), will have support added in DHE for better compatibility.

Additionally, a new type/namespace will be available offering some feature detection - this will be the first feature(s) to use those flags. The web UI will read those features then to detect if running against a deephaven JS API implementation that supports this feature.

When reverse=true, the start/end positions will be treated as offsets from the end of the table, rather than the beginning, as described in #1946. The API consumer will not need to update the viewport as the table's size changes, but will receive events with TableData with different offsets as fits the current table size and requested viewport.