Check keywords length during publish --dry-run

[txtyash]

This commit adds a length check on keywords in Cargo.toml. This will ensure that the user is aware of the limit on the length of keywords when running cargo publish --dry-run

[rustbot]

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[epage]

Number of keywords is a crates.io-specific rule while cargo is meant to be able to work with registries in addition to crates.io which can have their own rules.

Additionally, crates.io can change their validation rules at any point and cargo can't match. I'm going to close this in saying that more discussion is needed on #4377. If there is a reason we should keep this PR open, let us know!