Pin version of cargo-leptos

[stefnotch]

For my projects, I would like to have cargo-leptos behave more like a regular Rust dependency. This means

1. Specify its version in Cargo.toml, or a similar file.
2. Automatically have it installed, and accessible under some command. It is ok if the command ends up being a slightly longer cargo xtask leptos --help , or cargo leptos.rs --help

Number one would let me guarantee that every team member uses the same version of cargo-leptos, instead of "works on my machine". It also guarantees that the CI uses the correct version of cargo-leptos.

Number two means that onboarding new contributors is as frictionless as possible. Repeatedly having to tell people to install cargo-leptos (or trunk), and to keep it up-to-date, is a pain point for me.

And finally, if done well, I could juggle different projects that depend on different versions of cargo-leptos. Usually this happens when one project hasn't updated its dependencies just yet.