feat: Codegen feature flags (#28920)
This commit adds "deno_features" crate that contains definitions of all
unstable features in Deno.
Based on these definitions, both Rust and JS code is generated ensuring
that the two are always in sync.
In addition some of flag handling was rewritten to use the generated
definitions, instead of hand rolling these flag definitions.
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Co-authored-by: snek <snek@deno.com>
