Add support for ink-language-server

This commit add support for ink-language-server, which it does by
largely copying and pasting from the pure-language-server PR that was
merged recently.

The most interesting things to note are:
- ink-language-server is distributed upstream via npm, which is why we
  search through node_modules
- With some coaxing, it can be installed globally - which is why we
  search for a global binary.
- Ink is a funky language, and users will likely need to add
  initialization options.
- I am not incredibly familiar with vimscript; and I may not have done
  some of the buffer searching correctly.

supported-tools.md

@@ -207,6 +207,8 @@ formatting.
   * [write-good](https://github.com/btford/write-good)
 * Idris
   * [idris](http://www.idris-lang.org/)
+* Ink
+  * [ink-language-server](https://github.com/ephread/ink-language-server)
 * ISPC
   * [ispc](https://ispc.github.io/) :floppy_disk:
 * Java