Add license = "MIT" field to Cargo.toml

[LikeLakers2]

This PR adds a license = "MIT" field to Cargo.toml.

This does not change the license of bevy_ecs_tilemap - it is still the MIT license. However, this allows tools (such as crates.io or cargo-license) to better recognize the license used by this crate.

For example, currently crates.io cites this crate has having a non-standard license, as only license-file is specified and crates.io cannot detect if the file is indeed the MIT license. With this change, crates.io can (upon the next published update) detect that the license is indeed the MIT license.

---

P.S. I left the license-file field in because, according to the manifest format documentation for the include field:

The following files are always included:

• If a license-file is specified, it is always included.

and it is not otherwise clear if LICENSE would be included without some additional tinkering to Cargo.toml. As I am not sure if it's desired for the LICENSE file to be distributed, and it's unlikely that having both license and license-file would be considered an issue, I am taking the safe route - I am leaving the license-file field in.

[LikeLakers2]

@StarArawn Thanks! Do note that crates.io will only see this field when you next publish the crate - it doesn't read the field from the github repository.