R Backend highlighter

Summary:

• Use RKeywords to handle all the built-Ins of R

• Prepare a hard-coded list of reserved keywords and operators. Reference https://cran.r-project.org/doc/manuals/R-lang.pdf

• Use apropos('') command to get the built-Ins of R and then use reserved keywords and operators list to separate out functions from it

Reviewers: filipesaraiva

Subscribers: KDE Edu

Tags: KDE Edu, Cantor

Differential Revision: https://phabricator.kde.org/D8547