Automatically generate externals.js from package-lock.json

This commit adds a utility to parse package-lock.json and write the proper
  contents of externals.js to standard output. In addition, if the utility
  (src/helpers/pkglock_to_externals.litcoffee) is invoked with a --doc option,
  it instead emits a Markdown bulleted list of all of the external dependencies.

src/dyna3.litcoffee

@@ -6,22 +6,22 @@ When you load dyna3, you should initially see a three-dimensional coordinate sys
     import {threeLoaded, three as J3} from './externals.js'
 
     main = () ->
-       renderer = new J3.WebGLRenderer()
-       rwd = window.innerWidth
-       rht = window.innerHeight
-       renderer.setSize(rwd, rht)
-       document.body.appendChild renderer.domElement
+        renderer = new J3.WebGLRenderer()
+        rwd = window.innerWidth
+        rht = window.innerHeight
+        renderer.setSize(rwd, rht)
+        document.body.appendChild renderer.domElement
 
-       scene = new J3.Scene()
-       geometry = new J3.SphereBufferGeometry(1, 5, 5)
-       material = new J3.MeshBasicMaterial( {color: 0x0000ff} )
-       ball = new J3.Mesh(geometry, material)
-       scene.add ball
+        scene = new J3.Scene()
+        geometry = new J3.SphereBufferGeometry(1, 5, 5)
+        material = new J3.MeshBasicMaterial( {color: 0x0000ff} )
+        ball = new J3.Mesh(geometry, material)
+        scene.add ball
 
-       camera = new J3.PerspectiveCamera(75, rwd/rht, 0.1, 1000)
-       camera.position.z = 5
+        camera = new J3.PerspectiveCamera(75, rwd/rht, 0.1, 1000)
+        camera.position.z = 5
 
-       renderer.render(scene, camera)
+        renderer.render(scene, camera)
 
     threeLoaded.then(main)
 ```

src/helpers/pkglock_to_externals.litcoffee

@@ -0,0 +1,59 @@
+### External dependencies
+
+The dyna3 program depends on some externally-maintained JavaScript
+libraries/modules. The package uses npm to track these external dependencies.
+A module externals.js is automatically generated from the package_lock.json file
+created by npm to load the necessary modules at runtime.
+
+The generation is performed by pkglock_to_externals.litcoffee, which also
+records the main importable file within the library, as there does not seem
+to be a systematic way to generate that filename from the module name.
+```javascript
+
+    importable = { three: 'build/three.module.js' }
+
+```
+Currently, dyna3 makes use of the following external libraries:
+```javascript
+
+    if process.argv.length < 3
+        header =
+        """
+        /* External module loader for dyna3
+         * Generated automatically from package-lock.json by
+         * helpers/pkglock_to_externals.litcoffee
+         */
+
+        """
+        process.stdout.write header
+    fs = require 'fs'
+    pl = fs.readFileSync 'package-lock.json'
+    pdata = JSON.parse pl
+    for dep in Object.keys pdata.dependencies
+        if process.argv.length > 2
+            process.stdout.write "* #{dep}\n"
+        else
+            block =
+            """
+            export var #{dep};
+            export var #{dep}Loaded = new Promise(async function(resolve, reject) {
+                var success = false;
+                try {
+                    #{dep} = await import('https://cdn.jsdelivr.net/npm/#{dep}@#{pdata.dependencies[dep].version}/#{importable[dep]}');
+                    console.log('CDN import of #{dep} module OK');
+                    success = true;
+                } catch(err) {
+                    try {
+                        #{dep} = await import('./node_modules/#{dep}/#{importable[dep]}');
+                        success = true;
+                    } catch(err) {
+                    }
+                    console.log('Used local fallback for #{dep} module');
+                }
+                if (success) { resolve(); } else { reject(); }
+            });
+
+            """
+            process.stdout.write block
+
+```