feat(EnvVar): Environment Variables Store

[markovejnovic]

What does this PR do?

env_var.zig - Environment Variable Store

This PR adds a new mechanism to keep track of environment variables. Instead of loading them ad-hoc in various places, it uses a memoization strategy that loads the environment variable once, stores it in .data and then loads it as required.

The goal of this PR is to make the tracking and management of environment variables (and feature flags) cleaner and safer

Misc

• Made all .deinit() operations not take a const *T but now they all take *T on @taylordotfish's request
• Added POSIX support for uv_os_homedir
• Fixed a very minor leak and concurrency bug in fs.zig.

Future Improvements

• Some of the utilities in os.zig could potentially benefit from being memoized. Is C:\Windows really going to change during the execution of Bun? Probably not -- so it is worthwhile to memoize those too.
• Commonize uv_os_homedir between POSIX and Windows #23793

How did you verify your code works?

Hopefully regressions pass. This is a refactor and not much more.

Breaking Changes

• 💔 BUN_CONFIG_DNS_TIME_TO_LIVE_SECONDS has been changed to no longer accept negative inputs. The legacy behavior would allow you to pass negative numbers which would cause it to mean std.math.maxInt(32).