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/Cargo.toml

@@ -138,6 +138,7 @@ welcome.workspace = true
 workspace.workspace = true
 zed_actions.workspace = true
 zeta.workspace = true
+zlog.workspace = true
 zlog_settings.workspace = true
 workspace-hack.workspace = true