Fix R grammar for highlighting

[lionel-]

These changes fix a number of syntax higlighting issues caused by our TextMate grammar for R. Addresses #3688.

We are currently styling most of R variables as keywords or constants, which looks jarring with many themes. With the following changes we now style R variables with the conventional variable.parameter and variable.object scopes:

• The scopes of identifiers are renamed from constant.other to variable.object and from entity.name.tag to variable.parameter.

• To ensure these scopes are not overridden by a larger entity scope, we no longer use keyword.operator for bracket scopes, but meta.bracket. This is the appropriate scope prefix for large components in the code, see https://macromates.com/manual/en/language_grammars. This particular scope, meta.bracket, is not as popular as the ones discussed above but is used in the ObjC and Julia grammars.

  Similarly, we no longer scope function definitions as keyword but as meta.function.definition, the convention scope for these entities.

These changes allow colour themes to style R code much more consistently as they do other languages.

Before with Material:

After with Material:

Before with Monokai:

After with Monokai:

Note how these are less heavily styled.

We see kind of the opposite pattern with the default themes because they surprisingly seem to weigh more heavily regular text as opposed to keyword:

Before with Positron Light:

After with Positron Light:

Before with Positron Dark:

After with Positron Dark:

That said, this makes R styling consistent with how the theme works with other languages, here is how Positron Light and Dark style a C file for comparison:

[lionel-]

We've decided to merge this PR to make the R grammar consistent with the other languages. This will cause heavier styling of R code with the default Positron themes, but this is consistent with how we style other languages.

Note that Python will still not be as heavily styled but that is because we are currently not decorating variables with the variable scope: #3731