improve variadic funcs matching rules

[quasilyte]

Introduced a new matcher op: maybeVariadicCallExpr.
Unlike CallExpr, it doesn't unconditionally accept
both variadic and non-variadic calls. Instead, it only
accepts variadic calls if there are more than N arguments.

Take this pattern for example:

f($_, $_, $_, $*_)

Variadic call will be accepted only if there are more than
3 arguments passed to f.

f(a, b, c, 4)     // ok
f(a, b, c, xs...) // ok
f(a, b, xs...)    // NOT ok

Fixes #28