[WIP] Replace in project (#2984)

Targeting Preview of 09.27.
This is still pending several touchups/clearups:
- We should watch multibuffer for changes and rescan the excerpts. This
should also update match count.
- Closing editor while multibuffer with 100's of changed files is open
leads to us prompting for save once per each file in the multibuffer.
One could in theory save in multibuffer before closing it (thus avoiding
unnecessary prompts), but it'd be cool to be able to "Save all"/"Discard
All".

Release Notes:

- Added "Replace in project" functionality

crates/project/src/search.rs

@@ -231,7 +231,16 @@ impl SearchQuery {
             }
         }
     }
-    pub fn replacement<'a>(&self, text: &'a str) -> Option<Cow<'a, str>> {
+    /// Returns the replacement text for this `SearchQuery`.
+    pub fn replacement(&self) -> Option<&str> {
+        match self {
+            SearchQuery::Text { replacement, .. } | SearchQuery::Regex { replacement, .. } => {
+                replacement.as_deref()
+            }
+        }
+    }
+    /// Replaces search hits if replacement is set. `text` is assumed to be a string that matches this `SearchQuery` exactly, without any leftovers on either side.
+    pub fn replacement_for<'a>(&self, text: &'a str) -> Option<Cow<'a, str>> {
         match self {
             SearchQuery::Text { replacement, .. } => replacement.clone().map(Cow::from),
             SearchQuery::Regex {