cli: Convert package managers to lowercase during serialization

[acheroncrypto]

Problem

[toolchain.package_manager] field (added in #3328) is serialized using the enum defaults, meaning it will be in PascalCase format due to Rust's enum variants being PascalCase by convention.

For example, using anchor init program results in the following toolchain section:

[toolchain]
package_manager = "Yarn"

Using package_manager = "yarn" doesn't work:

Error: Unable to deserialize config: TOML parse error at line 2, column 19
  |
2 | package_manager = "yarn"

This is unintuitive, especially considering it's a lot more common to use lowercase when referring to package managers (and their commands) e.g. yarn or npm.

Summary of changes

Convert package manager names to lowercase during serialization.

[vercel]

@acheroncrypto is attempting to deploy a commit to the coral-xyz Team on Vercel.

A member of the Team first needs to authorize it.