Warn about potentially-confusing uses of function applications in patterns

[LPTK]

Direct function application syntax on a pattern hole is currently interpreted as higher-order matching. To extract a function, one should ascribe the hole with a function type. This can lead to confusing error, such as the following:

[error] Embedding Error: Unexpected expression in higher-order pattern variable argument: 42
[error]     case ir"$f(42)" => println(s"$f(42)")
[error]          ^

In this case, the error message should inform and guide users who just wanted to extract a function...