Provide a shorthand for writing out multiple `let` statements

At the moment you can write multiple `let` statements as follows:

```pkl
let (foo = 10)
let (bar = 42)
foo + bar
```

Provide a shorthand for writing out multiple `let` statements:

```pkl
let (foo = 10,
bar = 42)
foo + bar
```

This is a suggestion for notation. The main thing would be to reduce the number of `let` expressions required to define a bunch of values.

The issue names no files, tests, or entry points. Start by locating the parser and tests for `let` expressions, then determine how comma-separated bindings should be parsed and validated. Done means the proposed shorthand is supported while the existing multiple-`let` form continues to work.

Written by the indexing model from the issue text.