fix: detect reserved keyword in functions params

[TheAwiteb]

Check if the token is a keyword or reserved in the function parameters, if it's returns a Reserved error.

Ref: #1045

[schungx]

If it is a standard keyword then I think we need a more meaningful error message:

fn foo(abc, def, if x { c } else { d })  {}

We may want to say parameter expected on if?

For reserved keywords that should be fine...

Also check it after LexError might be better as the calls might be expensive...

[TheAwiteb]

Now this errors are returned

1: fn foo(default, def, if x { c } else { d })  {}
          ^ Syntax error: 'default' is a reserved keyword

and

1: fn foo(abc, def, if x { c } else { d })  {}
                    ^ Syntax error: Expecting name of a parameter but found 'if'

[schungx]

Only one thing... may not be a good idea to introduce a new error variant just for this case... as existing code do not know how to deal with this.

Can we shoehorn it into an existing error? Do you think?

How about VariableExpected?

[TheAwiteb]

How about VariableExpected?

I can do this, but I don't actually except a variable

[schungx]

A parameter is itself a variable... So technically speaking it is still kosher

[TheAwiteb]

Now, instead of

1: fn foo(abc, def, if x { c } else { d })  {}
                    ^ Syntax error: Expecting name of a parameter but found 'if'

it show

1: fn foo(abc, def, if x { c } else { d })  {}
                    ^ Syntax error: Expecting name of a variable