build: move to ESM only and minimize build

This commit moves the library to ESM only, and minimizes the build step by
writing the code in normal JavaScript with JSDoc comments. This reduces the need
for the build step to producing type declarations from the code, while still
maintaining type safety through checking the JSDoc comments.

BREAKING CHANGE: the library is now ESM-only, which means it cannot be used with
`require`.

.eslintignore

@@ -1,4 +1,4 @@
+*.d.ts
 .vscode
-dist
 node_modules
 website

.eslintrc.json

@@ -0,0 +1,19 @@
+{
+  "root": true,
+  "extends": ["eslint:recommended"],
+  "parserOptions": {
+    "ecmaVersion": "latest",
+    "sourceType": "module"
+  },
+  "env": {
+    "es2021": true
+  },
+  "overrides": [
+    {
+      "files": "test/*.js",
+      "env": {
+        "node": true
+      }
+    }
+  ]
+}

.gitignore

@@ -1,3 +1,3 @@
+*.d.ts
 .vscode
-dist
 node_modules

.prettierignore

@@ -1,5 +1,5 @@
+*.d.ts
 .vscode
-dist
 node_modules
 website/.docusaurus
 website/build

README.md

@@ -21,7 +21,16 @@ You can also use a CDN, such as [esm.sh](https://esm.sh), directly from a
 browser or Deno:
 
 ```js
-import { PitchDetector } from "https://esm.sh/pitchy@3";
+import { PitchDetector } from "https://esm.sh/pitchy@4";
+```
+
+Note that this package is
+[ESM-only](https://gist.github.com/sindresorhus/a39789f98801d908bbc7ff3ecc99d99c),
+meaning it can't be used with `require`. However, it can still be used from
+CommonJS code, albeit only in async contexts, using dynamic `import`:
+
+```js
+const { PitchDetector } = await import("pitchy");
 ```
 
 ## Examples and documentation