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-engine/src/eval.rs

@@ -698,6 +698,29 @@ pub fn eval_expression_with_input(
             input = eval_subexpression(engine_state, stack, block, input)?;
         }
 
+        elem @ Expression {
+            expr: Expr::FullCellPath(full_cell_path),
+            ..
+        } => match &full_cell_path.head {
+            Expression {
+                expr: Expr::Subexpression(block_id),
+                span,
+                ..
+            } => {
+                let block = engine_state.get_block(*block_id);
+
+                // FIXME: protect this collect with ctrl-c
+                input = eval_subexpression(engine_state, stack, block, input)?;
+                let value = input.into_value(*span);
+                input = value
+                    .follow_cell_path(&full_cell_path.tail, false)?
+                    .into_pipeline_data()
+            }
+            _ => {
+                input = eval_expression(engine_state, stack, elem)?.into_pipeline_data();
+            }
+        },
+
         elem => {
             input = eval_expression(engine_state, stack, elem)?.into_pipeline_data();
         }