Allow declare generics for function expressions

[MaxGraey]

var pass = <T>(a: T): T => a

[dcodeIO]

For a bit of background: I initially excluded this at the parser level because such a function isn't represented by a concrete value until fully resolved, so there's nothing to pass around, i.e. when calling a function with a generic function expression as an argument, or just store it in var pass. To me it looks like it's rather a type than a function, and its existence is strictly tied to the dynamic nature of JS. Not saying "no" ofc, but that's the reasoning so far :)

[MaxGraey]

Btw this code also produce error:

var pass = <T>(a: T): T => a;
pass<i32>(1);

Output:

ERROR TS2304: Cannot find name 'T'.

and later:

ERROR TS2349: Cannot invoke an expression whose type lacks a call signature. Type 'i32' has no compatible call signatures.

[dcodeIO]

Closing this issue as part of 2020 vacuum because it seems unlikely that we can solve this statically anytime soon due to a missing opportunity to represent it. If someone has an idea that doesn't involve costly runtime checks or otherwise including metadata, feel free to open a new issue or reopen this one.