Expose null_literal as a node

[afroozeh]

We need to expose null literals in parse trees, otherwise, it will not be possible to syntactically differentiate between some constructs. Example: currently, these two functions yield the same parse tree:

fun f() = null
fun f() { }

source_file
  function_declaration
    simple_identifier
    function_value_parameters
    function_body

With the changes in this PR, the first function will have a null_literal in the tree:

source_file
  function_declaration
    simple_identifier
    function_value_parameters
    function_body
      null_literal

Fixes #140

[afroozeh]

@fwcd Can you please check this PR? I have some other PRs that are stacked and like to merge them. I really don't want to fork the work on tree-sitter-kotlin, but expect some faster response time from you.
Also, it seems like someone posted an exact duplicate of this PR: #141
It doesn't matter for me which one gets merged, but let's merge one of them and move on with this one.

[wolfmanstout]

@fwcd Can you please check this PR? I have some other PRs that are stacked and like to merge them. I really don't want to fork the work on tree-sitter-kotlin, but expect some faster response time from you.

Also, it seems like someone posted an exact duplicate of this PR: #141

It doesn't matter for me which one gets merged, but let's merge one of them and move on with this one.

+1 I just closed my dupe PR. I hadn't seen this one when I made that but it's very similar.

[afroozeh]

@wolfmanstout please check the new commit. It adds the changes from your PR to this one.

[wolfmanstout]

@wolfmanstout please check the new commit. It adds the changes from your PR to this one.

Looks great, thanks.

[wolfmanstout]

@wolfmanstout please check the new commit. It adds the changes from your PR to this one.

Please also update the PR description to include "Fixes #140".

[fwcd]

Thanks!