Experimental ESM support for node >=13 (TypeStrong#1010)

* Initial commit

* fix exports declaration in package.json; update example readme

* add missing file to package files array

* Add missing .js extension to ESM resolver

* WIP to support EmitFlavor, which I'm going to undo since actually we don't need to support 2x emit flavors

* Add foo.js -> foo.ts resolution, remove compileEsm codepath, add support for experimental-specifier-resolution=node

* Add testing on node 14 to CI

* Add ESM header to README

* Add ESM test

* ESM loader factory accepts RegisterOptions

* Add copy of node's ESM loader to make diffing easier.

* Fix tests

* Fix tests

* type annotation in esm.mjs

* fix tests on ts 2.7

* Conditionally run esm tests on node >= 13

.github/workflows/continuous-integration.yml

@@ -7,7 +7,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        flavor: [1, 2, 3, 4]
+        flavor: [1, 2, 3, 4, 5, 6, 7]
         include:
           - flavor: 1
             node: 6
@@ -21,6 +21,15 @@ jobs:
           - flavor: 4
             node: 13
             typescript: typescript@next
+          - flavor: 5
+            node: 14
+            typescript: typescript@latest
+          - flavor: 6
+            node: 14
+            typescript: typescript@2.7
+          - flavor: 7
+            node: 14
+            typescript: typescript@next
     steps:
       # checkout code
       - uses: actions/checkout@v2

README.md

@@ -7,6 +7,10 @@
 
 > TypeScript execution and REPL for node.js, with source map support. **Works with `typescript@>=2.7`**.
 
+### *Experimental ESM support*
+
+Native ESM support is currently experimental. For usage, limitations, and to provide feedback, see [#1007](https://github.com/TypeStrong/ts-node/issues/1007).
+
 ## Installation
 
 ```sh