The motivation for this change was to avoid generating empty
config sections like
vim.cmd([[
]])
To make a config generation cleaner several helper functions introduced:
* `hasContent` have been moved to helpers
* `concatNonEmptyLines` joins strings (which has content) separated with
newlines
* `wrapVimscriptForLua` wraps a lua string for using in Vimscript, but
only if the string has content, otherwise empty string is returned
* `wrapLuaForVimscript` wraps Vimscript for using in lua, but only if
the string has content, otherwise empty string is returned
Added tests:
* testing that all possible config sections are present in the final
generated config
* testing that the config files generated by empty `files` definitions
don't have any content in it
