fix(init): example contracts: keep cdylib, exclude Cargo.lock

[chadoh]

This removes the cargo_toml dependency that I just added in #1296, opting for a simpler regex-based find-and-replace for the soroban-sdk dependency changes. Previous to #1296, we used toml_edit for all of these modifications, but did so in a way that ignored and overwrote important fields like soroban-sdk.features.

The toml_edit approach for editing the dependencies and dev_dependencies sections was quite verbose, and insufficient to what we needed, as the tests added in #1296 and this PR illustrate. Since we know what is in all the soroban-examples projects (for now), I thought maybe the regex approach was sufficient, for the time being. This could prove to be too brittle in the future, though, if one of the examples changes its format to not put all soroban-sdk settings on the same line.

The cargo_toml approach seemed like it should have been superior to both of these approaches, but it was changing more than it should have. For example, here's the starting Cargo.toml for the auth contract, prior to being modified by init:

[package] 
name = "soroban-auth-contract"
version = "0.0.0" 
edition = "2021" 
publish = false

[lib] 
crate-type = ["cdylib"]

[dependencies] 
soroban-sdk = { version = "20.3.1" }

[dev-dependencies] 
soroban-sdk = { version = "20.3.1", features = ["testutils"] }

With cargo_toml, the above was being changed to this:

[package] 
name = "soroban-auth-contract" 
edition = "2021" 
version = "0.0.0" 
publish = false
[dependencies.soroban-sdk] 
workspace = true
[dev-dependencies.soroban-sdk] 
features = ["testutils"] 
workspace = true

[lib] 
path = "src/lib.rs" 
name = "soroban_auth_contract" 
edition = "2021" 
crate-type = ["rlib"] 
required-features = []

Yikes! Where'd all that lib stuff come from?? That's not where edition goes! It already includes edition in the [package] section, why did it add it there, too?? And why on EARTH would it change lib.crate-type = ["cdylib"] to lib.crate-type = ["rlib"]?? Bad news! This means such crates won't even be considered Soroban contracts anymore! Wowee.