Create subcommands for 'ocaml-tree-sitter-gen-c' and ''ocaml-tree-sitter-gen-ocaml'

[mjambon]

As suggested here, let's make ocaml-tree-sitter the main command for everything, including:

• what ocaml-tree-sitter currently does: generate ocaml code from grammar.json
• simplify-grammar
• gen-ocaml, a bash script
• gen-c, a bash script
• possibly a combination of the above that does everything

Tasks include:

• finding good names for the subcommands
• locating the external executables to be called (the bash scripts): should we rely on $PATH to locate them or something else?
• adding subcommands to the cmdliner interface. See cmdliner cheatsheet for how to add subcommands.

Compatibility with the previous ocaml-tree-sitter command line is not important, since as far as I know, the only user is semgrep and all that code exists in the ocaml-tree-sitter repo (the semgrep build does not call ocaml-tree-sitter).

[mjambon]

Some of this work was done (#202) already: simplify is now a subcommand of ocaml-tree-sitter. What's left I think is to add subcommands for gen-c and gen-ocaml, which would call the commands ocaml-tree-sitter-gen-c and ocaml-tree-sitter-gen-ocaml respectively.