Add support for loading environment variables from .env file

[Leo5878]

I faced an issue where environment variables weren't being read correctly, even after exporting them. It was likely caused by some error in my system environment. Although the issue is now resolved, I added support for reading a .env file as a fallback. System environment variables still take precedence

[JasonShin]

Thanks for this, let me give it a try - this is interesting. I thought env crate would handle this already.

It looks good to me - I will quickly add a test and approve.

Thanks for your contribution!

[JasonShin]

Thanks for this! - this is now released as v0.22.0
https://github.com/JasonShin/sqlx-ts/releases/tag/v0.22.0