Add `process.EnvMap`, a platform-independent environment variable map

EnvMap provides the same API as the previously used BufMap (besides `putMove` and `getPtr`), so usage sites of `getEnvMap` can usually remain unchanged.

For non-Windows, EnvMap is a wrapper around BufMap. On Windows, it uses a new EnvMapWindows to handle some Windows-specific behavior:

- Lookups use Unicode-aware case insensitivity (but `get` cannot return an error because EnvMapWindows has an internal buffer to use for lookup conversions)
- Canonical names are returned when iterating the EnvMap

Fixes #10561, closes #4603