raise ParseError if assign to a non-variable or non-mutable-variable (#14405)

# Description
While reviewing #14388, I think we can make some improvement on parser.

For the following code:
```nushell
let a = 3
a = 10   # should be error
$a = 10 # another error
```
I think they can raise `ParseError`, so nushell doesn't need to move
forward compiling IR block.

# User-Facing Changes
```nushell
let a = 3
a = 10
```
Will raise parse error instead of compile error.

# Tests + Formatting
Added 1 test.

crates/nu-command/tests/commands/mut_.rs

@@ -148,3 +148,14 @@ fn def_should_not_mutate_mut() {
     assert!(actual.err.contains("capture of mutable variable"));
     assert!(!actual.status.success())
 }
+
+#[test]
+fn assign_to_non_mut_variable_raises_parse_error() {
+    let actual = nu!("let x = 3; $x = 4");
+    assert!(actual
+        .err
+        .contains("parser::assignment_requires_mutable_variable"));
+
+    let actual = nu!("mut x = 3; x = 5");
+    assert!(actual.err.contains("parser::assignment_requires_variable"));
+}