Use shorter SPDX @license headers

[aomarks]

This PR changes the license headers on the .ts source files in the following ways:

1. Adds a @license annotation, so that tools like Terser and Closure compiler will automatically know to preserve these license headers by default.

2. Uses a new, more concise style of header with the SPDX-License-Identifier property, which is approved by Google's open source office (e.g. it's used by Lit).