Nail down cargo-script syntax proposal about lang-team

[joshtriplett]

cc @epage

[joshtriplett]

Spelling out some possibilities discussed in the meeting:

• One option would be to say that the only thing that interprets the ``` blocks is the interpreter on the shebang line, and if it doesn't understand a block it should reject it. Cargo could then reject anything other than cargo, or in the future cargo-lock, and rustc would ignore everything.
• Another option would be for rustc to have an allowlist of what it ignores, and error on anything else.

What to do here depends somewhat on where we see future extensions going. If we expect this to only be used by scripts, the former option seems reasonable: the script interpreter can take responsibility for the namespace. If we expect this to be potentially used by other Rust tools as well (e.g. an ```analyzer block to pass info to rust-analyzer, or a block for some other tool), then we may want to have rustc manage the allowlist.

Some folks in the meeting objected to the idea of having broader usage/extensions that weren't for scripts. Some folks objected to the idea of not having an allowlist somewhere (either in rustc or cargo).

I'm hoping we can reach a consensus to narrowly allow cargo for now, which gives us the future option of either dropping or extending the safelist.