Compare keyword tables

[kwalrath]

https://dart.dev/guides/language/language-tour#keywords and https://github.com/dart-lang/sdk/issues/43072#issuecomment-675324473 don't quite agree — for example, if is a reserved word in the language tour, but isn't bold in one of its occurrences in the issue. (It is bold in the other, so I think that the language tour's table is correct there.)

It might be good to carefully go through both tables to make sure that they agree and that the links in the language tour cover all uses of each keyword. (E.g. if in the language tour points to https://dart.dev/guides/language/language-tour#if-and-else, which doesn't have any reference to conditional imports. Maybe that's OK.)