Select applicable positions for lsp_ext methods more leniently (#32272)

Closes https://github.com/zed-industries/zed/issues/27238

Release Notes:

- Fixed `editor::SwitchSourceHeader` and
`editor::ExpandMacroRecursively` not working with text selections

crates/text/src/selection.rs

@@ -26,6 +26,7 @@ impl Default for SelectionGoal {
 }
 
 impl<T: Clone> Selection<T> {
+    /// A place where the selection had stopped at.
     pub fn head(&self) -> T {
         if self.reversed {
             self.start.clone()
@@ -34,6 +35,7 @@ impl<T: Clone> Selection<T> {
         }
     }
 
+    /// A place where selection was initiated from.
     pub fn tail(&self) -> T {
         if self.reversed {
             self.end.clone()