fix error when exporting consts with type signatures in modules (#14118)

Fixes #14023

# Description

- Prevents "failed to find added variable" when modules export constants
  with type signatures:

```nushell
> module foo { export const bar: int = 2 }
Error: nu::parser::unknown_state

  × Internal error.
   ╭─[entry #1:1:21]
 1 │ module foo { export const bar: int = 2 }
   ·                     ─────────┬────────
   ·                              ╰── failed to find added variable
```

- Returns `name_is_builtin_var` errors for names with type signatures:

```nushell
> let env: string = "";
Error: nu::parser::name_is_builtin_var

  × `env` used as variable name.
   ╭─[entry #1:1:5]
 1 │ let env: string = "";
   ·     ─┬─
   ·      ╰── already a builtin variable
```

tests/repl/test_modules.rs

@@ -117,6 +117,10 @@ fn export_consts() -> TestResult {
     run_test(
         r#"module spam { export const b = 3; }; use spam b; $b"#,
         "3",
+    )?;
+    run_test(
+        r#"module spam { export const b: int = 3; }; use spam b; $b"#,
+        "3",
     )
 }