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 ...

Cargo.lock

@@ -18287,6 +18287,7 @@ dependencies = [
  "workspace-hack",
  "zed_actions",
  "zeta",
+ "zlog",
  "zlog_settings",
 ]
 
@@ -18599,6 +18600,7 @@ dependencies = [
 name = "zlog"
 version = "0.1.0"
 dependencies = [
+ "anyhow",
  "log",
  "workspace-hack",
 ]