keymap: Allow modifiers as keys (#12047)

It is sometimes desirable to allow modifers to serve as keys themselves
for the purposes of keybinds. For example, the popular keybind in
jetbrains IDEs `shift shift` which opens the file finder.

This change treats modifers in the keymaps as keys themselves if they
are not accompanied by a key they are modifying.

Further this change wires up they key dispatcher to treat modifer change
events as key presses which are considered for matching against
keybinds.


Release Notes:

- Fixes #6460

---------

Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

crates/gpui/src/platform/keystroke.rs

@@ -94,6 +94,27 @@ impl Keystroke {
             }
         }
 
+        //Allow for the user to specify a keystroke modifier as the key itself
+        //This sets the `key` to the modifier, and disables the modifier
+        if key.is_none() {
+            if shift {
+                key = Some("shift".to_string());
+                shift = false;
+            } else if control {
+                key = Some("control".to_string());
+                control = false;
+            } else if alt {
+                key = Some("alt".to_string());
+                alt = false;
+            } else if platform {
+                key = Some("platform".to_string());
+                platform = false;
+            } else if function {
+                key = Some("function".to_string());
+                function = false;
+            }
+        }
+
         let key = key.ok_or_else(|| anyhow!("Invalid keystroke `{}`", source))?;
 
         Ok(Keystroke {
@@ -186,6 +207,10 @@ impl std::fmt::Display for Keystroke {
             "right" => '→',
             "tab" => '⇥',
             "escape" => '⎋',
+            "shift" => '⇧',
+            "control" => '⌃',
+            "alt" => '⌥',
+            "platform" => '⌘',
             key => {
                 if key.len() == 1 {
                     key.chars().next().unwrap().to_ascii_uppercase()
@@ -241,6 +266,15 @@ impl Modifiers {
         }
     }
 
+    /// How many modifier keys are pressed
+    pub fn number_of_modifiers(&self) -> u8 {
+        self.control as u8
+            + self.alt as u8
+            + self.shift as u8
+            + self.platform as u8
+            + self.function as u8
+    }
+
     /// helper method for Modifiers with no modifiers
     pub fn none() -> Modifiers {
         Default::default()