fix: add .js extension to imports and fix package.json

This fixes two problems with the ESM build of this module.

1. The `package.json` that contains `{ "type": "module" }` wasn't being included in the npm tarball
2. When running in an ESM environment, `import foo from './bar'` does not work, you have to specify the extension

The fix for the first is simple, add the cjs/esm `package.json` files to the `files`
array in the project `package.json`.

The second fix is harder.  If you just add the `.js` extension to the source files,
typescript is happy but ts-node is not, and this project uses ts-node to run the
tests without a compile step.

Typescript does not support importing `*.ts` and will not support adding `*.js` to
the transpiled output - microsoft/TypeScript#16577

ts-node thought this was a bug in Typescript but it turns out not.  Their suggestion
to use `ts-node/esm` breaks sourcemap support because `source-map-support/register`
is not esm - TypeStrong/ts-node#783

There is a PR against ts-node to add support for resolving `./foo.js` if `./foo.ts`
or `./foo` fails but it seems to have stalled - TypeStrong/ts-node#1361

Given all of the above, the most expedient way forward seemed to just be to add
a shell script that rewrites the various `import` statements in the esm output
to add the `.js` extension, then if the ts-node PR ever gets merged the script
can be backed out.

Fixes beaugunderson#147

.fix-esm-imports.sh

@@ -0,0 +1,4 @@
+#!/bin/sh
+
+# Search for "from './baz'" and transform to "from './baz.js'"
+find dist/esm -type f -name *.js -exec sed -i -e -E "s/from '(\.\/.*)'/from '\1.js'/g" {} \;

package.json

@@ -21,7 +21,7 @@
   },
   "scripts": {
     "docs": "documentation build --github --output docs --format html ./ip-address.js",
-    "build": "rm -rf dist/* && tsc -p tsconfig.json && tsc -p tsconfig-esm.json && ./.fix-package-types.sh",
+    "build": "rm -rf dist/* && tsc -p tsconfig.json && tsc -p tsconfig-esm.json && ./.fix-package-types.sh && ./.fix-esm-imports.sh",
     "prepublishOnly": "npm run build",
     "release": "release-it",
     "test-ci": "nyc mocha",
@@ -53,7 +53,8 @@
   "files": [
     "dist/**/*.js",
     "dist/**/*.ts",
-    "dist/**/*.map"
+    "dist/**/*.map",
+    "dist/**/*.json"
   ],
   "repository": {
     "type": "git",