It should not be possible to use keywords as identifiers

The following code runs, but should not.

```r
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: https://github.com/dgkf/R/blob/5c13c4950991c821aa5fd2255751d3eabc3316a1/src/grammar/grammar.pest#L189-L192

We have to pay attention to the localizations.

Start in src/grammar/grammar.pest around lines 189–192, where identifiers and keywords are defined. Check how keyword localization is represented before adjusting the grammar. The examples using function, fn, and if should no longer run as assignments, including under the relevant localizations.

Written by the indexing model from the issue text.