feat: Improve tool.pixi.project detection logic

[olivier-lacroix]

@tdejager , as discussed, an improved detection of pixi project table in pyproject.toml , encapsulating all non-pixi pyproject.toml logic used during init into a struct.

plus minor cleanup (e.g. typo)

[ruben-arts]

Although it's not ready for review. I did it anyways and it looks good to me 😄

[tdejager]

Same here, going to merge!