It should not be possible to use keywords as identifiers

[sebffischer]

The following code runs, but should not.

function = 1
fn = 1
if = 1

I believe the best way to fix this is to just encode this in the grammar. I.e. exclude the keywords from being an identifier:

R/src/grammar/grammar.pest

Lines 189 to 192 in 5c13c49

	symbol = _{ symbol_with_backticks | symbol_ident }
	symbol_with_backticks = _{ "`" ~ symbol_backticked ~ "`" }
	symbol_backticked = ${ ( !"`" ~ ANY )* }
	symbol_ident = ${ ( LETTER | OTHER_SYMBOL | "_" | "." ) ~ ( ASCII_ALPHANUMERIC | "_" | "." )* }

We have to pay attention to the localizations.