Make Addin for selection styling respecting line breaks in general

[lorenzwalthert]

Previously, the Addin did not support selecting the last line to style as a full line as shown below in the sense that it resulted in a line break removed and resulted in invalid R code.

resulted in

1 + 1 q

For the last line selected for styling, one had to select a finite col2 value to prevent this.

This resulted in the correct styling

1 + 1
q

This PR removes the constraint of having to select the code as in the second example (i.e. a non-finite col2 value is no longer needed) and closes #519.

[codecov-io]

Codecov Report

Merging #520 into master will increase coverage by 0.05%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #520      +/-   ##
==========================================
+ Coverage   90.12%   90.18%   +0.05%     
==========================================
  Files          39       39              
  Lines        1732     1732              
==========================================
+ Hits         1561     1562       +1     
+ Misses        171      170       -1

Impacted Files	Coverage Δ
R/addins.R	0% <0%> (ø)	⬆️
R/utils.R	88.88% <0%> (+5.55%)	⬆️

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