General improvements

[TeddiO]

These are general updates to improve the library, remove footguns and remove unneccesary libs.

• Remove regex usage & regex library
• Added STEAM_AUTH_HTTP_TIMEOUT with a default value of 15 seconds to give better control around the outbound lifecycle for requests to Steam.
• Replaced manual query string construction with idiomatic use of url.Values and string methods, simplifying the logic and reducing potential bugs.
• Introduced a httpDoer interface and a global httpClient variable, allowing for dependency injection and easier testing of HTTP interactions.
• Improved error handling and validation in ValidateResponse, including stricter checks on claimed identity and SteamID extraction.
• Added a new test file gosteamauth_test.go with a fakeHTTPClient to mock HTTP responses, providing thorough unit tests for ValidateResponse, including success, failure, network error, timeout, and identity mismatch scenarios.
• Add GitHub Actions CI job for running said tests.