Reorganize rust crates into a flat crates directory, simplify some CI steps (#4496)

* Move all rust crates (except lib) into crates dir, w/o nesting

* Remove stale path from .gitattributes

* Rename lib.rs files for easier navigation

* Rename mod.rs file for easier navigation

* Fix emscripten-version path

* Fix fixtures dir paths

* Use the default rustfmt settings

* Don't use nightly on CI

cli/src/templates/binding_test.js

@@ -1,9 +0,0 @@
-const assert = require("node:assert");
-const { test } = require("node:test");
-
-const Parser = require("tree-sitter");
-
-test("can load grammar", () => {
-  const parser = new Parser();
-  assert.doesNotThrow(() => parser.setLanguage(require(".")));
-});