tst-tabcount wót Tensor

This extension places a tab number on each tab rendered by the (excellent) TreeStyleTab plugin.

This functionality was previously supplied by a few lines of CSS in the TST configuration, but has been broken since 4.0.1 (see: https://github.com/piroor/treestyletab/issues/3486 )

This extension is an implementation of the TST author's suggestion to use the TST API instead. It has zero user-configurable options, as it hardcodes my own preferred CSS style (orange on black).