Do not search for nested arrow functions outside of arrow functions

[sirbrillig]

#334 caused a performance regression. This line was now performing a large scope search for every token when searching for a variable's scope in order to find the scope of nested arrow functions, even if the current variable was not inside an arrow function.

In this diff we prevent searching for the arrow function's scope opener if we are not inside an arrow function. That does not speed up searching for the arrow function scope itself but it will prevent this cost being paid for every variable.

Fixes #340

[sirbrillig]

Hm... This is not accurate. We can't know if we're in an arrow function without knowing the closing scope. Let me try something else.