Introduce condition expression to IfExpression

[tesuji]

The current reference syntax of IfExpression is not clear enough:

I had to read more on explanation that:

The form of an if expression is a condition expression, followed by a consequent block, any number of else if conditions and blocks, and an optional trailing else block

So I think introduce a condition expression in syntax might make sense.
Prior art in other languages:

• C++ if syntax has condition expression: https://en.cppreference.com/w/cpp/language/if
• Swift: https://docs.swift.org/swift-book/ReferenceManual/Statements.html#grammar_if-statement

[ehuss]

Closing as we now have a Conditions rule via #1740.