[WIP] refactor: Use writer without allocating entire output

[LauH1987]

This builds on top of my other PR (#7). When debugging the issue in that PR I noticed that the current implementation would allocate the entire output as a string even when using the to_writer function. This PR tries to avoid this.

Having never implemented something like this before I took a lot of inspiration from the serde_json crate. The goal is to eventually be able to use the serializer by itself for instance using serde_transcode.

I understand that env-files are rarely very large so serializing to memory before outputting is usually not an issue.

There are still some things missing like doc strings and a public Serializer::new function, so I'm marking this PR as a draft. I didn't want to sink more time into this before getting feedback.

[LauH1987]

I personally find the expressions like

let SeqState::NotSeq = self.seq_state else { ... }

quite hard to comprehend, so alternatively using the std::ops::Not trait this could be written like:

if matches!(ser.seq_state, SeqState::NotSeq).not() { ... }