SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
This required changing the load_sync API to take a LoadRequest instead
of just a URL. Since HTMLScriptElement was the only (non-test) user of
this API, it didn't seem useful to instead add an overload of load_sync
for this.
