Move logo generator script to data-raw directory

- Move R/.logo.R to data-raw/logo_generator.R to avoid R CMD check warnings
- .logo.R file name was causing 'invalid file name' error during build
- data-raw directory is already in .Rbuildignore, so it's excluded from package

Author: mrchypark

.Rbuildignore

@@ -14,3 +14,5 @@
 ^\.github$
 ^data-raw$
 ^cran-comments\.md$
+^CRAN-SUBMISSION$
+^\.logo\.R$