GH-43960: [R] fix str_sub binding to properly handle negative end values

[coussens]

First-time contributor here, so let me know where I can improve!

Rationale for this change

The str_sub binding in arrow was not handling negative end values properly. The problem was two-fold:

1. When end values were negative (and less than the start value, which might be positive), str_sub would improperly return an empty string.
2. When end values were < -1 but the end position was still to the right of the start position, str_sub failed to return the final character in the substring, since it did not account for the fact that end is counted exclusively in the underlying C++ function (utf8_slice_codeunits), but inclusively in R.

See discussion/examples at #43960 for details.

What changes are included in this PR?

1. The removal of lines from r/R/dplyr-funcs-string.R that previously set end= 0 when start < end, which meant if the user was counting backwards from the end of the string (with a negative end value), an empty string would [wrongly] be returned. It appears that the case that the previous code was trying to address is already handled properly by the underlying C++ function (utf8_slice_codeunits).
2. Addition of lines to r/R/dplyr-funcs-string.R in order to account the difference in between R's inclusive end and C++'s exclusive end when end is negative.
3. The addition of a test (described below) to r/tests/testthat/test-dplyr-funcs-string.R to test for these cases.

Are these changes tested?

Yes, I ran all tests in r/tests/testthat/test-dplyr-funcs-string.R, including one which I added (see attached commit), which explicitly tests the case where end is negative (-3) and less than the start value (1). This also tests the case where end < -1 and to the right of the start position.

Are there any user-facing changes?

No.

This PR contains a "Critical Fix". Previously:

• When end values were negative (and less than the start value, which might be positive), str_sub would improperly return an empty string.
• When end values were < -1 but the end position was still to the right of the start position, str_sub failed to return the final character in the substring, since it did not account for the fact that end is counted exclusively in the underlying C++ function (utf8_slice_codeunits), but inclusively in R.

• GitHub Issue: [R] stringr binding for str_sub() silently mishandles negative start/stop values #43960

[github-actions]

⚠️ GitHub issue #43960 has been automatically assigned in GitHub to PR creator.