Add OneOf shape to fix else (#7385)

# Description

fixes #7384 

This is a stop-gap fix until we remove type-directed parsing. With this,
you can create a `OneOf` shape that can be one of a list of syntax
shapes. This gives you a little more control than you get with `Any`,
allowing you to add `Block` without breaking other parsing rules.

# User-Facing Changes

`else` block will no longer capture variables as it will now use a block
instead of a closure.

# 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.

src/tests/test_conditionals.rs

@@ -59,3 +59,19 @@ fn if_elseif3() -> TestResult {
 fn if_elseif4() -> TestResult {
     run_test("if 2 > 3 { 5 } else if 6 < 7 { 4 } else { 8 } ", "4")
 }
+
+#[test]
+fn mutation_in_else() -> TestResult {
+    run_test(
+        "mut x = 100; if 2 > 3 { $x = 200 } else { $x = 300 }; $x ",
+        "300",
+    )
+}
+
+#[test]
+fn mutation_in_else2() -> TestResult {
+    run_test(
+        "mut x = 100; if 2 > 3 { $x = 200 } else if true { $x = 400 } else { $x = 300 }; $x ",
+        "400",
+    )
+}