Adding mapping prefix (and possibly suffix too?!)

[xarunoba]

We can add another declaration called prefix within the env declaration that is used to group prefixed env vars. For example:

const Config = struct {
    name: []const u8, // Maps to "APP_NAME" env var
    port: u32, // Maps to "APP_PORT" env var
    debug: bool = false, // Maps to "APP_DEBUG" env var
    timeout: ?f32 = null, // Maps to "APP_TIMEOUT" env var

    const env = .{
        .name = "NAME",
        .port = "PORT",
        .debug = "DEBUG",
        .timeout = "TIMEOUT",

        const prefix = "APP_"
        // const suffix = "_TEST"
    };
};

const config = try env_struct.load(Config, allocator);

The initial plan is to apply this prefix recursively for nested structs. I'll probably also include support for suffix since, why not?