Working with R source code in lunarvim

1. configuring ~/.config/lvim/config.lua for R and trouble
2. mention mason
3. important R key combinations
* ,rf to start R running
* ,aa to source the whole file
4. important general key combinations
* gd go to definition of symbol
* gr see references
* _lf reformat
* _; go to lunarvim control centre
* _ls show local symbols (functions, global vars)
* _xx show 'trouble'
5. demonstration with ~/sinclair_sine.R