Add a new load_with_encoding function to handle files with various encodings.

Modified `Buffer::reload` in `buffer.rs` to use this new function, allowing Zed
to open files with any encoding in UTF-8 mode. Files with characters that
are invalid in UTF-8 will have those bytes replaced with the � character.

Add comments and documentation.

crates/copilot/src/copilot.rs

@@ -1193,6 +1193,7 @@ async fn get_copilot_lsp(fs: Arc<dyn Fs>, node_runtime: NodeRuntime) -> anyhow::
 #[cfg(test)]
 mod tests {
     use super::*;
+    use encoding::Encoding;
     use gpui::TestAppContext;
     use util::path;
 
@@ -1406,6 +1407,10 @@ mod tests {
         fn load_bytes(&self, _cx: &App) -> Task<Result<Vec<u8>>> {
             unimplemented!()
         }
+
+        fn load_with_encoding(&self, _: &App, _: &'static dyn Encoding) -> Task<Result<String>> {
+            unimplemented!()
+        }
     }
 }