Allow $env and mutable records to be mutated by = (closes #7110) (#7318)

# Description

Closes #7110. ~~Note that unlike "real" `mut` vars, $env can be deeply
mutated via stuff like `$env.PYTHON_IO_ENCODING = utf8` or
`$env.config.history.max_size = 2000`. So, it's a slightly awkward
special case, arguably justifiable because of what $env represents (the
environment variables of your system, which is essentially "outside"
normal Nushell regulations).~~
EDIT: Now allows all `mut` vars to be deeply mutated using `=`, on
request.

# User-Facing Changes

See above.

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-parser/src/parser.rs

@@ -1997,28 +1997,22 @@ pub fn parse_full_cell_path(
         );
         error = error.or(err);
 
-        if !tail.is_empty() {
-            (
-                Expression {
-                    ty: head.ty.clone(), // FIXME. How to access the last type of tail?
-                    expr: Expr::FullCellPath(Box::new(FullCellPath { head, tail })),
-                    span: full_cell_span,
-                    custom_completion: None,
+        (
+            Expression {
+                // FIXME: Get the type of the data at the tail using follow_cell_path() (or something)
+                ty: if !tail.is_empty() {
+                    // Until the aforementioned fix is implemented, this is necessary to allow mutable list upserts
+                    // such as $a.1 = 2 to work correctly.
+                    Type::Any
+                } else {
+                    head.ty.clone()
                 },
-                error,
-            )
-        } else {
-            let ty = head.ty.clone();
-            (
-                Expression {
-                    expr: Expr::FullCellPath(Box::new(FullCellPath { head, tail })),
-                    ty,
-                    span: full_cell_span,
-                    custom_completion: None,
-                },
-                error,
-            )
-        }
+                expr: Expr::FullCellPath(Box::new(FullCellPath { head, tail })),
+                span: full_cell_span,
+                custom_completion: None,
+            },
+            error,
+        )
     } else {
         (garbage(span), error)
     }