fix: make it of "module" type

When running tests, node 20 complains:
"
SyntaxError: Cannot use import statement outside a module
"
(interestingly enough, previous versions of node do not complain)

Even prior to using node v20, there has been confusion, as seen from e.g.
b380b88
(Note on why this is related: modules _do_ require use of file extensions
in import statements)

Hopefully this change makes it clean for all (node, ts-node
and dependents importing this package)

Author: swiing

example/index.ts

@@ -1,5 +1,5 @@
-import BoolArray from '../src/bitarray';
-import { log as _, logHeader as _$ } from './util';
+import BoolArray from '../src/bitarray.js';
+import { log as _, logHeader as _$ } from './util.js';
 
 import { bit } from '@bitarray/typedarray';
 

package.json

@@ -2,6 +2,7 @@
   "name": "@bitarray/es6",
   "version": "1.1.2",
   "description": "An ES6 BitArray class for easy and native-like operations on sequences of bits",
+  "type": "module",
   "typings": "./dist/esm/src/bitarray.d.ts",
   "exports": {
     ".": {
@@ -24,13 +25,13 @@
     "build:cjs": "tsc -p tsconfig.cjs.json",
     "build:esm": "tsc -p tsconfig.esm.json",
     "build:types": "tsc --emitDeclarationOnly -p tsconfig.json",
-    "example": "node --loader ts-node/esm example",
+    "example": "ts-node --esm --project ./tsconfig.esm.json ./example/index.ts",
     "postbuild": "bash ./scripts/postbuild.sh",
     "prepare": "npm run build",
     "prepublishOnly": "npm run test",
     "style:fix": "prettier {example,src,test}/**/*.ts --write",
     "style:check": "prettier {example,src,test}/**/*.ts --check",
-    "test": "node --loader ts-node/esm test"
+    "test": "ts-node --esm --project ./tsconfig.esm.json ./test/index.ts"
   },
   "author": "swiing",
   "license": "MIT",

src/bitarray.ts

@@ -25,16 +25,8 @@
 
 import BitTypedArray from '@bitarray/typedarray';
 
-// missing js extension fails dependent applications
-// but adding js extension fails example/test of the library itself
-// => quick and dirty solution for now: inline import.
-//
-// import { base64MIMEChars, base64UrlChars } from './alphabet';
-const lettersAndDigits =
-  'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
-
-const base64MIMEChars = lettersAndDigits + '+/';
-const base64UrlChars = lettersAndDigits + '-_';
+import { base64MIMEChars, base64UrlChars } from './alphabet.js';
+
 const alphabetLengthErrorMsg = "Alphabet's length must be a power of 2";
 
 // I could leverage _views from @bitarray/typedarray, or create a new one here.

test/index.ts

@@ -1,4 +1,4 @@
-import suite from './suite';
-import test from './test';
+import suite from './suite.js';
+import test from './test.js';
 
 test(suite);

test/suite.ts

@@ -1,4 +1,4 @@
-import BitArray from '../src/bitarray';
+import BitArray from '../src/bitarray.js';
 
 const len = 42; // choose whatever value