Add default-run = "hx" to helix-term/Cargo.toml

[Omnikar]

Following the addition of xtask, cargo run has multiple possible targets, necessitating the usage of the less convenient cargo run --bin hx to run Helix during development. This allows cargo run to be used to run hx.

[archseer]

Does this need to sit inside helix-term or on the workspace level Cargo.toml?

[Omnikar]

If it were added to the top-level Cargo.toml, it'd mean adding a [package] section to it, which you can't actually do without having to add the required [package] fields like name etc., so I do think it makes more sense to put it in helix-term.