Since we only use the `Vec` created by the `collect` in the loop below,
there is no reason for the collect. Removing the collect prevents an
unnecessary allocation of the `Vec`, and allows the loop instead to
lazily compute the stringified chunks during iteration

Having a type for DIF validation errors will give us more flexible error handling abilities.

This will make redundant the existing warning in the funciton used to validate a DIF's size, allowing us to delete that warning. This change will help make the code more readable and maintainable.

Previously, we only logged an easy-to-miss warning when we had to skip
uploading a debug file because it was too big. Now, we instead fail the
entire upload loudly, to ensure users do not miss this important
information.

Resolves #2313

Recently, `cargo build` started failing on my local system, with an error related to `memmap2`. Upgrading the dependency to the latest version fixes the problem.

Prevent an unnecessary copy of all sourcemap files being uploaded from
being stored in memory, by using an `Arc` for the `contents` field of
`SourceFile`.

Fixes #2342

This custom deserializer can be easily replaced with
`derive(Deserialize)`.

Symbolic version `12.13.3` includes a change, which will
reduce the memory usage of sourcemap uploads, in some cases very
significantly.

ref #2344

Replace the unmaintained crate `dotenv` with the maintained fork
`dotenvy`.

Fixes #2350