This PR adds support for throwing named errors with associated error
explanations. In particular, it adds elaborators for the syntax defined
in #8649, which use the error-explanation infrastructure added in #8651.
This includes completions, hovers, and jump-to-definition for error
names.
Note that another stage0 rebuild will be required to define explanations
using `register_error_explanation`.
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Co-authored-by: Joachim Breitner <mail@joachim-breitner.de>
Co-authored-by: Marc Huisinga <mhuisi@protonmail.com>
