fix: introduce CloudEventV1 and CloudEventV03 interfaces (#194)

This extracts all of the attributes from a `CloudEventVX` that
are not generated by the constructor (id and specversion) into their
own `CloudEventVXAttributes` interface which the `CloudEventVX`
interface extends. This allows TS devs to optionally provide `id`
and `specversion` with proper autocompletion.

Additionally, I have added a union type, `CE` in `cloudevent.ts` which
represents any of `CloudEventV1`, `CloudEventv03`, `CloudEventV1Attributes`
and `CloudEventV03Attributes` interfaces.

Finally, this commit changes all of the user facing API to be `.ts` instead
of `.js` files.

The existing documentation in `./docs` was removed. It should be replaced with generated
HTML from tsdocs, pending some other method of publishing API documentation. That will
come as a separate, docs-only PR.

Fixes: https://github.com/cloudevents/skd-javascript/issues/188

Signed-off-by: Lance Ball <lball@redhat.com>

Author: lance

Diff for: .gitignore

@@ -5,8 +5,10 @@ npm-debug.log*
 yarn-debug.log*
 yarn-error.log*
 
-# Generated typedefs
+# Generated files
 *.d.ts
+index.js
+lib
 
 # Runtime data
 pids