Support for custom paths for all ocaml binaries

[eberejustin]

It would be nice if it were possible to point the extension to a /non/opam/path/bin to look for all the binaries it needs (ocaml-lsp-server, dune, ocamlformat, etc.).

I think you could do some pointing to individual executables with this extension.

My use-case is that on some environments I don't use opam and have pre-built binaries installed at some path using the system's package manager.

Originally posted by @code-ghalib in #1039 (comment)

Currently facing issues of earlybird is not installed in the current sandbox. Hint: $ opam install earlybird even though I have it installed, just without opam