Implement PRs checks to validate that curated license expressions are actually valid SPDX

[sschuberth]

    PR validation is a great idea. we have an spdx library that should be able to do that work so putting together an action should be straight forward. 

cc @nellshamrell

Originally posted by @jeffmcaffer in #7937 (comment)

[nellshamrell]

Hi @sschuberth! I am no longer working on Clearly Defined (I've moved to another project at MSFT). Tagging @mpcen.

[tyevil]

Ok

[SteveL-MSFT]

The current SPDX check isn't correctly identifying Unicode-3.0 which is listed here https://spdx.org/licenses/Unicode-3.0.html

and blocking my PR #28735

The Unicode-3.0 identifier is already used multiple times within the curated files