7010: Update ungrammar for const block patterns r=matklad a=Veykril
Fixes#6848
Adds const blocks and const block patterns to the AST and parses them.
Blocked on https://github.com/rust-analyzer/ungrammar/pull/17/, will merge that PR there once this one gets the OK so I can remove the local ungrammar dependency path and fix the Cargo.lock.
Co-authored-by: Lukas Wirth <lukastw97@gmail.com>
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