Change iftype to use elseif instead of elseiftype as next keyword.

[jemc]

This PR changes one of the keywords used in the new iftype syntax introduced in #1855.

Specifically, it changes the use of elseiftype to elseif. This is more consistent with ifdef, which uses elseif and not elseifdef.

The detail of what keyword to use for this wasn't included in the RFC, and I doubt very many people are using this feature yet given that the plan was to not formally announce it until the rest of the RFC was complete, so it seems like fair game to change.

Also, there were no tests using elseiftype, so I added a test using elseif.

[jemc]

Other than consistency, this also gives a better visual alignment of the conditions:

Before:

iftype A <: T1 then foo()
elseiftype A <: T2 then bar()
else baz()
end

After:

iftype A <: T1 then foo()
elseif A <: T2 then bar()
else baz()
end