`set::env_set_var` is unsafe in the 2024 edition.
All functions calling it are marked as unsafe.

- dotenv moved to dotenvy folder
- dotenvy-macros crate created for runtime macros
- temporary 2024 edition to check for unsafe `set_var` usage
- new API with `EnvLoader` builder, `load`, and `load_and_modify`
- `load` attribute macro that is thread-safe and async-compatible
- many examples added
- some irrelevant tests removed

This naming better reflects the fact that we are configuring a loader
with an optional path, with deferred I/O.

This removes the `EnvVar` variant which wraps `VarError`.
The internals of `VarError` are now variants in `Error`.

This makes it more ergonomic for users who are propogating errors to
know which varaible is truly missing.

This reintroduces `var` as a copy of `std::env::var` with more info.
The signature is different because in this crate, we only handle valid
UTF-8.

Calling `var` now gives the key name in the error.

This change `EnvMap` to a newtype with a `var` function that returns
`Result`.
This mirrors the behaviour of `dotenvy::var`.

This makes the CLI feature aligned with the attribute macro.

The optional example is also updated to be similar to the CLI.
`Path::exists` is no longer used because of a potential race condition.

Error messages are also adjusted.

This creates an internal `ParseBufError` to handle line parsing errors
without the path context.

Loading a file reports the file path error if an attempt was made to
read from it and it is not found.

`EnvLoader::path` is a setter that can be used to specify the path.
If `with_reader` is used, the path can be specified for error context.

If both reader and path are present, load will use the reader as input.

Now that we only use paths and there is also reader input, we don't need
to test with temp file creating and deleting.

The tests are also rewritten for clarity.

Tests are also moved from separate files to the `EnvLoader` module.

The integration tests are no longer being used.
Testing is done at lower levels and will be stored as modules.

Unit tests run in parallel and may clobber each other.
temp-env is now a dev dependency to ensure that unit tests run in
isolated enviornments.

Some tests are added for `load` and `load_and_modify`.

This is to build with clap_builder v4.5.17, which requires 17.4.0.
1.74.0 came out November 2023, so this is less than one year ago.

`cmd.exec` was accessible to the Windows code path, while also being
unreachable do to `exit(1)`.

We consistenly use "env file" across the repo now.

`override` argument named to `override_` because it is a keyword.