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-command/tests/commands/mut_.rs

@@ -36,18 +36,6 @@ fn capture_of_mutable_var() {
     assert!(actual.err.contains("capture of mutable variable"));
 }
 
-#[test]
-fn mut_a_field() {
-    let actual = nu!(
-        cwd: ".", pipeline(
-        r#"
-        mut y = {abc: 123}; $y.abc = 456; $y.abc
-        "#
-    ));
-
-    assert_eq!(actual.out, "456");
-}
-
 #[test]
 fn mut_add_assign() {
     let actual = nu!(
@@ -95,3 +83,63 @@ fn mut_divide_assign() {
 
     assert_eq!(actual.out, "4");
 }
+
+#[test]
+fn mut_path_insert() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        mut y = {abc: 123}; $y.abc = 456; $y.abc
+        "#
+    ));
+
+    assert_eq!(actual.out, "456");
+}
+
+#[test]
+fn mut_path_insert_list() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        mut a = [0 1 2]; $a.3 = 3; $a | to nuon
+        "#
+    ));
+
+    assert_eq!(actual.out, "[0, 1, 2, 3]");
+}
+
+#[test]
+fn mut_path_upsert() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        mut a = {b:[{c:1}]}; $a.b.0.d = 11; $a.b.0.d
+        "#
+    ));
+
+    assert_eq!(actual.out, "11");
+}
+
+#[test]
+fn mut_path_upsert_list() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        mut a = [[[3] 2] 1]; $a.0.0.1 = 0; $a.0.2 = 0; $a.2 = 0; $a | to nuon
+        "#
+    ));
+
+    assert_eq!(actual.out, "[[[3, 0], 2, 0], 1, 0]");
+}
+
+#[test]
+fn mut_path_operator_assign() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        mut a = {b:1}; $a.b += 3; $a.b -= 2; $a.b *= 10; $a.b /= 4; $a.b
+        "#
+    ));
+
+    assert_eq!(actual.out, "5");
+}