Consider adding "open" and "close" field names for strings

[DavisVaughan]

tree-sitter-r/grammar.js

Lines 481 to 491 in c8b6e5f

	_single_quoted_string: $ => seq(
	'\'',
	optional(field("content", alias($._single_quoted_string_content, $.string_content))),
	'\''
	),
	_double_quoted_string: $ => seq(
	'"',
	optional(field("content", alias($._double_quoted_string_content, $.string_content))),
	'"'
	),

So you can get at whether it is a single or double quoted string by field name

Using "open" and "close" is similar to what we do in other places