Add FreeBSD support

[jfouquart]

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Currently, to run on FreeBSD you need to:

install cask, npm and cargo.

launch: cargo install tree-sitter-cli

For npm, as the tgz doesn't exist for the moment you have to create a node_modules/tree-sitter-cli directory and copy the cli.js, dsl.d.ts, install.js, package.json, LICENSE files from the official tree-sitter/cli/npm/tree-sitter repository and also tree-sitter from the directory where cargo installed the binary

Of course you have to add ('berkeley-unix (list ".so")) in elisp-tree-sitter/lisp/tree-sitter-load.el to be able to load the .so

and that's it

If the FreeBSD build becomes integrated by default, all the prerequisites become superfluous