Support BASIC string with percent sign (%)

[udhos]

This BASIC code:

echo '10 print "%"' | basgo-build > a.go && go run a.go

Produces this invalid Go code:

fmt.Print(`%!`(MISSING))

It is possible to work-around this defect by escaping % with %%:

echo '10 print "%%"' | basgo-build > a.go && go run a.go