build(aio): auto-fill width/height to all image tags

Parse all `<img>` tags, during doc-gen, and insert the width and height of
the sourceed image, if neither are already specified.

Warnings are reported if the `<img>` tag has no `src` attribute or the image
cannot be loaded.

The work is done in the `addImageDimensions` post-processor, which must be
configured with a `basePath` so that it knows where to find the images.

Closes #15888

aio/tools/transforms/angular-base-package/services/getImageDimensions.js

@@ -0,0 +1,6 @@
+const { resolve } = require('canonical-path');
+const sizeOf = require('image-size');
+
+module.exports = function getImageDimensions() {
+  return (basePath, path) => sizeOf(resolve(basePath, path));
+};