Do not require mut on ParsedPath::element_mut (bevyengine#8891)

# Objective

`ParsedPath` does not need to be mut to access a field of a `Reflect`.
Be that access mutable or not. Yet `element_mut` requires a mutable
borrow on `self`.

## Solution

- Make `element_mut` take a `&self` over a `&mut self`.

bevyengine#8887 fixes this, but this is a major limitation in the API and I'd
rather see it merged before 0.11.

---

## Changelog

- `ParsedPath::element_mut` and `ParsedPath::reflect_element_mut` now
accept a non-mutable `ParsedPath` (only the accessed `Reflect` needs to
be mutable)

crates/bevy_reflect/src/path.rs

@@ -371,7 +371,7 @@ impl ParsedPath {
  ///
  /// See [`element_mut`](Self::element_mut) for a typed version of this method.
  pub fn reflect_element_mut<'r, 'p>(
- &'p mut self,
+ &'p self,
  root: &'r mut dyn Reflect,
  ) -> Result<&'r mut dyn Reflect, ReflectPathError<'p>> {
  let mut current = root;
@@ -402,7 +402,7 @@ impl ParsedPath {
  ///
  /// See [`reflect_element_mut`](Self::reflect_element_mut) for an untyped version of this method.
  pub fn element_mut<'r, 'p, T: Reflect>(
- &'p mut self,
+ &'p self,
  root: &'r mut dyn Reflect,
  ) -> Result<&'r mut T, ReflectPathError<'p>> {
  self.reflect_element_mut(root).and_then(|p| {