(cargo-pgrx init) Use toml serializer to write config.toml

[passcod]

Writing the toml by formatting strings meant that on Windows, backslashes in paths weren't escaped, outputting invalid toml. This uses the toml crate and the existing ConfigToml struct to read/write the file intead.

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Also in the readme it says

If a new minor Postgres version is released in the future you can simply run cargo pgrx init [args] again, and your local version will be updated, preserving all existing databases and configuration.

That was a bit confusing to me, as e.g. running:

$ cargo pgrx init --pg12 download --pg13 download

Followed by

$ cargo pgrx init --pg13 download --pg14 download

Would "erase" the pg12 checkout from the config.toml file, even though the files remained there.

I took that as accidental and fixed it (ie, we read the config and modify it) in this PR, but of course can revert that bit if it was intentional.

[workingjubilee]

That seems to be correct!