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-cmd-lang/src/core_commands/const_.rs

@@ -21,7 +21,7 @@ impl Command for Const {
             .required("const_name", SyntaxShape::VarWithOptType, "constant 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 constant value",
             )
             .category(Category::Core)

crates/nu-cmd-lang/src/core_commands/if_.rs

@@ -20,7 +20,7 @@ impl Command for If {
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("if")
             .input_output_types(vec![(Type::Any, Type::Any)])
-            .required("cond", SyntaxShape::Expression, "condition to check")
+            .required("cond", SyntaxShape::MathExpression, "condition to check")
             .required(
                 "then_block",
                 SyntaxShape::Block,

crates/nu-cmd-lang/src/core_commands/let_.rs

@@ -22,7 +22,7 @@ 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",
             )
             .category(Category::Core)

crates/nu-cmd-lang/src/core_commands/mut_.rs

@@ -22,7 +22,7 @@ impl Command for Mut {
             .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",
             )
             .category(Category::Core)

crates/nu-cmd-lang/src/core_commands/while_.rs

@@ -21,7 +21,7 @@ impl Command for While {
         Signature::build("while")
             .input_output_types(vec![(Type::Nothing, Type::Nothing)])
             .allow_variants_without_examples(true)
-            .required("cond", SyntaxShape::Expression, "condition to check")
+            .required("cond", SyntaxShape::MathExpression, "condition to check")
             .required(
                 "block",
                 SyntaxShape::Block,