Add assistant_context_editor crate (#23429)

This PR adds a new `assistant_context_editor` crate.

This will ultimately house the `ContextEditor` so that it can be
consumed by both `assistant` and `assistant2`.

For the purposes of this PR, we just introduce the crate and move some
supporting constructs to it, such as the `ContextStore`.

Release Notes:

- N/A

crates/assistant_slash_command/src/assistant_slash_command.rs

@@ -262,6 +262,67 @@ impl SlashCommandOutputSection<language::Anchor> {
     }
 }
 
+pub struct SlashCommandLine {
+    /// The range within the line containing the command name.
+    pub name: Range<usize>,
+    /// Ranges within the line containing the command arguments.
+    pub arguments: Vec<Range<usize>>,
+}
+
+impl SlashCommandLine {
+    pub fn parse(line: &str) -> Option<Self> {
+        let mut call: Option<Self> = None;
+        let mut ix = 0;
+        for c in line.chars() {
+            let next_ix = ix + c.len_utf8();
+            if let Some(call) = &mut call {
+                // The command arguments start at the first non-whitespace character
+                // after the command name, and continue until the end of the line.
+                if let Some(argument) = call.arguments.last_mut() {
+                    if c.is_whitespace() {
+                        if (*argument).is_empty() {
+                            argument.start = next_ix;
+                            argument.end = next_ix;
+                        } else {
+                            argument.end = ix;
+                            call.arguments.push(next_ix..next_ix);
+                        }
+                    } else {
+                        argument.end = next_ix;
+                    }
+                }
+                // The command name ends at the first whitespace character.
+                else if !call.name.is_empty() {
+                    if c.is_whitespace() {
+                        call.arguments = vec![next_ix..next_ix];
+                    } else {
+                        call.name.end = next_ix;
+                    }
+                }
+                // The command name must begin with a letter.
+                else if c.is_alphabetic() {
+                    call.name.end = next_ix;
+                } else {
+                    return None;
+                }
+            }
+            // Commands start with a slash.
+            else if c == '/' {
+                call = Some(SlashCommandLine {
+                    name: next_ix..next_ix,
+                    arguments: Vec::new(),
+                });
+            }
+            // The line can't contain anything before the slash except for whitespace.
+            else if !c.is_whitespace() {
+                return None;
+            }
+            ix = next_ix;
+        }
+        call
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use pretty_assertions::assert_eq;