Ensure linter logic allows for R>=4 raw strings

[MichaelChirico]

Some linters (e.g. current paste_sep_linter and paste_to_string_linter) rely on assumptions about STR_CONST nodes that are designed with R<4 in mind, where R"(a string)" is not available.

We should take a pass over linters to ensure robustness to this new style of literal strings.

[AshesITR]

Note: as.character(parse(text = 'r"(a)"', keep.source = FALSE)) == "a" seems like a base R way to extract the string without eval().

[MichaelChirico]

Here's all the linters that might need to be addressed:

grep -Flr "STR_CONST" R | awk -F/ '{print $2}'

object_usage_linter.R
extraction_operator_linter.R
expect_type_linter.R
literal_coercion_linter.R
strings_as_factors_linter.R
condition_message_linter.R
path_linters.R
object_name_linters.R
single_quotes_linter.R
paste_linter.R
expect_s3_class_linter.R
get_source_expressions.R
expect_named_linter.R
make_linter_from_regex.R
unneeded_concatenation_linter.R
missing_package_linter.R
sprintf_linter.R
yoda_test_linter.R

[MichaelChirico]

One more TODO here: I think unquote() can be entirely replaced by the new as.character(parse()) approach.