Rework logrx to minimize dependencies

[bms63]

Background

logrx depends on stringr and stringi to do a lot of string manipulation. This means folks have to install both packages for logrx to work.

Definition of Done

• Rework all logrx functions to use base R string functions
• Remove all dependencies on stringr and stringi across files
• Redo any documentation or vignettes to remove stringr and stringi needs
• All tests should pass.
• Make a note of this rework in the news file