Change maximumLevel to availableLevels in implicit tiling extension

[lilleyse]

This is targeting a staging branch where we'll be pushing revisions to the 3D Tiles Next extensions during the next couple of weeks as we prepare for 3D Tiles 1.1. The staging branch will be merged into main once everything stabilzes.

Changes maximumLevel to availableLevels for consistency with subtreeLevels. Note that maximumLevel is an index whereas availableLevels is a length, so availableLevels is equivalent to maximumLevel + 1.

See discussion in #617

[lilleyse]

I'm going to merge this into the staging branch where the full set of subtree changes can be reviewed together later.