Get Executing Script's Path, from 'RStudio', 'RGui', 'Rterm' and 'Rscript' (Command-Line / / Terminal), and When Using 'source'

Determine the full path of the executing script. Works when running a line or selection from an open R script in 'RStudio' and 'RGui', when using 'source' and 'sys.source' and 'debugSource' ('RStudio' exclusive) and 'testthat::source_file', and when running R from the Windows command-line / / Unix terminal.