zlog: Support configuring log levels with env var (#28544)

Reimplemented logic from `env_logger` to parse log configuration from
environment variables.

Had to re-implement instead of using `env_filter` crate that
`env_logger` uses, as it does not export the information required to
integrate it.


Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/zed/src/main.rs

@@ -195,6 +195,7 @@ fn main() {
         return;
     }
 
+    zlog::init_from_env();
     if stdout_is_a_pty() {
         init_stdout_logger();
     } else {