cli: Allow opening non-existent paths (#43250)

Changes are made to `parse_path_with_position`:
we try to get the canonical, existing parts of
a path, then append the non-existing parts.

Closes #4441

Release Notes:

- Added the possibility to open a non-existing path using `zed` CLI
  ```
  zed path/to/non/existing/file.txt
  ```

Co-authored-by: Syed Sadiq Ali <sadiqonemail@gmail.com>

crates/zlog/README.md

@@ -0,0 +1,15 @@
+# Zlog
+
+Use the `ZED_LOG` environment variable to control logging output for Zed
+applications and libraries. The variable accepts a comma-separated list of
+directives that specify logging levels for different modules (crates). The
+general format is for instance:
+
+```
+ZED_LOG=info,project=debug,agent=off
+```
+
+- Levels can be one of: `off`/`none`, `error`, `warn`, `info`, `debug`, or
+  `trace`.
+- You don't need to specify the global level, default is `trace` in the crate
+  and `info` set by `RUST_LOG` in Zed.