(feat): Unpack extensions into complex function types #8

The more in-depth we use extensions, the more likely is is that we need custom extensions. New `UnpackExtensions` function in the high package makes this easy. Low level models have been updated to support feature fully and docs added in README and examples as well.

Signed-off-by: Dave Shanley <dave@quobix.com>

datamodel/high/base/external_doc_test.go

@@ -35,6 +35,7 @@ x-hack: code`
 
 	wentLow := highExt.GoLow()
 	assert.Equal(t, 2, wentLow.URL.ValueNode.Line)
+	assert.Len(t, highExt.GetExtensions(), 1)
 }
 
 func ExampleNewExternalDoc() {