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 fixes an issue where `undefined.foo = "bar"` would throw a
ReferenceError instead of a TypeError as undefined was also used for
truly unresolvable references (e.g. `foo() = "bar"`). I also made the
various error messages here a bit nicer, just "primitive value" is not
very helpful.
