preserve elseif syntax in parser

[JeffBezanson]

Part of #21774. I think this change is particularly useful, since

• macros like @static should be able to handle elseif chains; elseifs are arguably part of the same outermost expression and so should all be evaluated in the same stage
• much nicer representation from show
• it's very minimally breaking, since it only changes the interior of if blocks, and :elseif expressions are semantically identical to if expressions

[vtjnash]

just curious: why didn't you keep this as "word"? Seems like that elseif may be more accurate in the error message, although the actual impact is quite minimal either way?

[JeffBezanson]

Previously word was always if, so I did this to keep the same error message. I could go either way on this though.

[tkelman]

https://github.com/JuliaLang/julia/blob/09f059a2d719cd1f7d052ccd40782200d4998097/doc/src/devdocs/ast.md#block-forms needs updating, right?