Fix typing error in Parser

[mendrixsebas]

We get this error when compiling our project when we import easy: Fix error TS2526: A 'this' type is available only in a non-static member of a class or interface This forces us to add "skipLibCheck": true to our tsconfig.json compilerOptions, which we rather not do. This PR fixes the typing.

[mendrixsebas]

@woutervanbakel mentioning you for visibility

[woutervanbakel]

@mendrixsebas Thanks! Looks like the tests depend on the old implicit behavior, so we might run into the same on our side. Luckily it’ll fail at build time, not runtime.

I think I can merge this once you fixed the unit tests