feat(spdx-licenses): implement is_valid_license_string SPDX expression parser

PHP matches the SPDX license-expression grammar with a single recursive
PCRE pattern ((?(DEFINE) subpatterns plus (?&name) recursion), which the
regex crate cannot express, so port it as a hand-written recursive-descent
parser instead. licenseid/licenseexceptionid dictionary lookups use
longest-match-first, guarded against accepting a short entry that is only
a coincidental prefix of a longer identifier run (e.g. "DOC" prefixing
"DocumentRef-..."), which a length-sorted greedy match alone would
misparse.

Un-ignores the two validating_array_loader tests that were blocked on
this todo!().

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