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 *
https://sqlite.org/lang_expr.html
The entry point to using expressions, parse_expression(), is not used
by SQL::Parser in this commit. But there's so much here that it's easier
to grok as its own commit.
