feat: Open new terms in last term's CWD

Closes: pop-os#251

This patch implements an optional (but enabled by default) feature for
opening new terminals using the focused terminal's working directory.
The code to retrieve the CWD is largely based on Alacritty's
implementation of the same feature.

I added a new direct dependency, Rustix, which was already included
transitively. I opted for Rustix instead of libc since I could avoid a
use of `unsafe` with Rustix. Rustix's type guarantees are also stronger
than libc's.

src/config.rs

@@ -238,6 +238,8 @@ pub struct Config {
     pub syntax_theme_light: String,
     pub focus_follow_mouse: bool,
     pub default_profile: Option<ProfileId>,
+    /// Open new terminal with the current working directory of the focused term
+    pub current_working_directory: bool,
 }
 
 impl Default for Config {
@@ -262,6 +264,7 @@ impl Default for Config {
             syntax_theme_light: COSMIC_THEME_LIGHT.to_string(),
             use_bright_bold: false,
             default_profile: None,
+            current_working_directory: true,
         }
     }
 }