Require commas after all but the last field assignment in class literals

[yorickpeterse]

Please describe the bug

While taking a look at https://github.com/dusty-phillips/inko-http/ I noticed that we don't error for code like this:

SomeClass {
  @field1 = value1
  @field2 = value2
}

We shouldn't allow this as it can lead to unclear syntax and potentially result in the parser parsing things differently. For example, I'm not sure what happens with code like this:

SomeClass {
  @field1 = value1 @field2 = value2
}

To resolve this, we should just require a comma after every assignment, except for the last assignment (i.e. no comma is needed if the next character is }).

Please list the exact steps necessary to reproduce the bug

Create test.inko with these contents:

class Foo {
  let @a: Int
  let @b: Int
}

class async Main {
  fn async main {
    Foo {
      @a = 10
      @b = 20
    }
  }
}

Then run inko check test.inko. This should fail with a parser error, but it doesn't.

Operating system

Fedora Silverblue 38

Inko version

main

Rust version

1.68.2