Let parser work without section headers

[Hubro]

Currently the parser is strict about section headers. This is fine for Robot files, since they are invalid without section headers anyway.

However, it would be nice if the parser could be used for quick snippets as well. For example, currently the parser would be unable to parse this snippet:

Test Something
    Hello World

Because it's missing a section header.

The parser should probably be updated to allow a raw list of test cases if there are no section headers.

Something like (pseudo code):

{
  rules: {
    // ...

    source_file: $ => seq(
      optional(/\s+/), // Allows whitespace before the first section
      choice(
        repeat($.section),
        repeat($.keyword_definition),
      )
    ),

    // ...
  }
}