Address emscripten dependency in tree-sitter-xpath

It must have slipped my mind that emscripten is an external dependency for tree-sitter-xpath. I'd originally hoped to address this by making it a package dependency. I believe that may be possible (despite the tree-sitter documentation suggesting otherwise), but if I recall I deferred addressing it before other tooling dust had settled. Either way it should be addressed, either by inclusion in package install or by explicit documentation.

Begin by checking how tree-sitter-xpath currently handles package installation and compare it with the tree-sitter documentation on emscripten. Determine whether emscripten can be included as a package dependency or must be documented as a prerequisite; done means one of those paths is implemented and verified.

Written by the indexing model from the issue text.