Fix some yield expressions not recognized

[PPazderski]

Existing parser fails to recognize some yield expressions. Especially yielding the result of a method call or the result of a conditional expression.
E.g. the following snippet currently fails:

int x = switch (5) {
    default -> {
        yield getX();
    }
};

The problem I found is that yield is not a keyword but just a restricted identifier or since Java 17 called contextual keyword. Javaparser currently tries to parse the statement as VariableDeclarationExpression.

This pull request is a minimal change to fix the problem of yield statements not recognized. For a better solution I think javaparser would need to better different between Identifier and TypeIdentifier according to https://docs.oracle.com/javase/specs/jls/se17/html/jls-3.html#jls-3.8
Then the VariableDeclarationExpression would start with a type which cannot be the name "yield".
If a class could be named "yield" a statement like yield x = null; is ambiguous and could be a local variable declaration plus initialization or a yield statement whose expression is an assignment.
But I expect this to be a notably larger and riskier change.

PS: if someone wants to try the SwitchExprTest.yieldYield example, the following code compiles:

public class Main {

    public enum X {
        yield,
    }

    public static void main(String[] args) {
        X yield = X.yield;
        yield = switch (yield) {
        default -> {
            yield yield = yield;
        }
        case yield -> {
            yield yield == yield ? yield : yield;
        }
        };
    }
}

[jlerbsc]

Thank you for your contribution.

[PPazderski]

Thank you for merging my pull requests.