Fix: Environment variables do not overwrite Config.toml options

[GrumiumXD]

Closes #2429

This is my first PR on this project and it turned out a bit bigger than expected. So I am not sure about some of the decisions I made.

• Removed separator("_") from the Environment source setup, because this is intended for nested structures.
• Added convert_case(Case::Kebab) to the Environment source setup to match the casing serde expects.
• Removed "[leptos-options]" from toml string to let the settings be parsed as root level, otherwise the settings from the file and the ones set by the environment variables would be considered in different structures, therefore not affecting each other. Because of this get_config_from_str() now returns a LeptosOptions object instead of a ConfFile.

The test cases gave me some troubles as I had some inconsistent results. I assume (because tests are executed in parallel) that sometimes variables set in one test could interfere with a test that does not expect them (now that they could actually overwrite settings originated from a file source).
I added temp-env as a dev dependency to address my issues. Afterwards the tests were working consistently again and I added a new test to explicitly test the override functionality.

Because I changed the parsing from ConfFile to LeptosOptions, I think the serde::Deserialize trait could actually be removed from the ConfFile struct. Any opinions on that?

[benwis]

Nice work! I'd love to remove the Deserialize struct if that's possible.

[GrumiumXD]

I was just talking about getting rid of the Deserialize trait for the ConfFile struct, but getting rid of the whole ConfFile struct and only working with LeptosOptions everywhere sounds even better, I guess.
I will see what I can do.

[benwis]

I was just talking about getting rid of the Deserialize trait for the ConfFile struct, but getting rid of the whole ConfFile struct and only working with LeptosOptions everywhere sounds even better, I guess. I will see what I can do.

To be honest that is my bad wording, We have both of those because of how the config crate works. At any rate, I think this looks good, and can merge and see if anything breaks