fix(init): don't add two dev_dependencies sections

[chadoh]

When initializing a new project with soroban contract init, we need to
clean up the Cargo.toml of example projects a little. We need to make
sure soroban-sdk is always inherited from the workspace, and we need
to make sure there's no profile section.

We had a couple issues with the old logic.

1. It was adding a dev_dependencies section, with an underscore, which
   is deprecated. If the Cargo.toml already had a dev-dependencies
   section, it would add a second dev_dependencies, which cargo would
   complain about.
2. The new setting would only set { workspace = true }, even if the
   old one included other details like features = ['testutils'].

This adds a new crate, cargo_toml, which allows us to parse and
manipulate Cargo.toml files in a more sophisticated way, which allows us
to make sure we get the dev-dependencies section even if it's called
dev_dependencies. It also makes it straightforward to add the correct
inherited dependency.

Known limitations

The format of the new Cargo.toml is a little unconventional:

[package]
name = "soroban-account-contract"
edition = "2021"
version = "0.0.0"
publish = false
[dependencies.soroban-sdk]
workspace = true
[dev-dependencies.ed25519-dalek]
version = "1.0.1"

[dev-dependencies.rand]
version = "0.7.3"

[dev-dependencies.soroban-sdk]
features = ["testutils"]
workspace = true

[willemneal]

Looks good to me. Love a crate that makes everything smoother.

[willemneal]

We could use some tests, but this seems straight forward enough not to warrant one before the merge. Perhaps we can add more tests for the init in a future PR.