i92642 added the ability to use certain keywords as variable names (eg. name = 1, line = "hi"),
but also caused a regression where "Line Input" is broken in single-line If statements.
This patch fixes that by allowing Then and Else to also be the start-of-line tokens expected to
immediately preceed the "Line" token in order for that "Line" token to be recognized a keyword instead
of a variable name. Also added FVT spreadsheet tests for "Line" as both a variable name and as "Line Input".

Patch by: me

(cherry picked from commit f8a51d0f)