Allow configuring esy path

[bbenne10]

Presently, src/analyze/BuildSystem.re (and maybe a few other places, I haven't checked too deeply) does not allow a configurable path to find esy and assumes that esy is in $PATH (or system equivalent) on launch.

This prevents a few different workflows:

• esy installed globally, but with a unique npm prefix set to a directory that's not in a default $PATH (This is what I'm seeing. It's installed in ~/.local/bin, which I must push into $PATH by hand on OSX)
• esy installed globally without overriding prefix: This just means that /usr/local isn't in $PATH
• esy installed locally (and thus in <project_root>/node_modules/.bin): This is also pretty common and works amazingly with direnv (but unless that shell also starts rls, we're in trouble.

I am, admittedly, blissfully unaware of how hard this would be. It looks like BuildSystem.inferPackageManager and BuildSystem.detectFull are the only two places that Error("Couldn't get esy version") is thrown, but I have no idea where those functions are used (or if there're any other non-qualified uses of esy)