Keep shared internal constants in a central location

[MichaelChirico]

I was looking around for such a file, seems we don't have one yet. Would apply to other internal constants as well, e.g.

lintr/R/expect_type_linter.R

Line 76 in bb7669a

base_types <- c(

lintr/R/object_name_linter.R

Line 1 in bb7669a

object_name_xpath <- local({

Follow-up, or I'll make that sweep in this PR?

Originally posted by @MichaelChirico in #2046 (comment)

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As usual the only trouble is naming (for the file). 'aaa.R' and 'constants.R' are the natural choices, any other thoughts/suggestions?

[AshesITR]

They could also go in zzz.R because they are only needed at runtime so collation doesn't matter.