Consider `field("opening_delimiter", $._open_parenthesis)` in `call_arguments` and friends

[DavisVaughan]

It is often useful to locate the open/close parentheses/brackets/brackets2 delimiters to be able to, say, ensure that the cursor is between them. This is currently awkward without giving them field names, so we should consider giving call, subset, and subset2 consistent field names for this. @kevinushey does that make sense in your head?

i.e. a call's arguments node is structured as

    call_arguments: $ => seq(
      $._open_parenthesis, 
      repeat($._argument), 
      $._close_parenthesis
    ),

instead move to:

    call_arguments: $ => seq(
      field("opening_delimiter", $._open_parenthesis), 
      repeat($._argument), 
      field("closing_delimiter", $._close_parenthesis)
    ),

So then we could try to grab them by name rather than by offset, i.e. the much cleaner:

node.child_by_field_name("open")

[kevinushey]

Seems reasonable to me!