Switch let/let-env family to init with math expressions (#8545)

# Description

This is an experiment to see what switching the `let/let-env` family to
math expressions for initialisers would be like.

# User-Facing Changes

This would require any commands you call from `let x = <command here>`
(and similar family) to call the command in parentheses. `let x = (foo)`
to call `foo`.

# 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

> **Note**
> from `nushell` you can also use the `toolkit` as follows
> ```bash
> use toolkit.nu # or use an `env_change` hook to activate it
automatically
> toolkit check pr
> ```

# 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/tests/test_parser.rs

@@ -27,7 +27,7 @@ impl Command for Let {
             .required("var_name", SyntaxShape::VarWithOptType, "variable name")
             .required(
                 "initial_value",
-                SyntaxShape::Keyword(b"=".to_vec(), Box::new(SyntaxShape::Expression)),
+                SyntaxShape::Keyword(b"=".to_vec(), Box::new(SyntaxShape::MathExpression)),
                 "equals sign followed by value",
             )
     }
@@ -1555,7 +1555,7 @@ mod input_types {
 
         fn signature(&self) -> nu_protocol::Signature {
             Signature::build("if")
-                .required("cond", SyntaxShape::Expression, "condition to check")
+                .required("cond", SyntaxShape::MathExpression, "condition to check")
                 .required(
                     "then_block",
                     SyntaxShape::Block,