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>

script/debug-cli

@@ -1,3 +1,3 @@
 #!/usr/bin/env bash
 
-cargo build; cargo run -p cli -- --foreground --zed=target/debug/zed "$@"
+cargo build -p zed && cargo run -p cli -- --foreground --zed=${CARGO_TARGET_DIR:-target}/debug/zed "$@"