ast, parser: implement simple AST poisoning

[nedpals]

This PR implements a feature similar to #7236 but in a minimal, non-destructive way:

• Error node is NodeError (See: [0])
• Does not replace ast.File's errors but only references them with an array index where the error is located.
• Only used in parser methods that return an empty ast.Stmt{} or ast.Expr{} element.

This feature is crucial in order to eliminate most of the crashes VLS is experiencing on every change due to an invalid memory error on the nodes that were identified as unknown ast.Stmt / unknown ast.Expr.

Notes

[0] - NodeError is used due to some conflict when using Error as the struct name when including it into a sumtype (I suspect it's a naming conflict with the builtin Error)

<v folder>/vlib/v/ast/ast.v:1437:16: error: type `Error` has no field or method `pos`
 1435 |         SelectorExpr, SizeOf, SqlExpr, StringInterLiteral, StringLiteral, StructInit, Type, TypeOf,
 1436 |         UnsafeExpr {
 1437 |             return expr.pos
      |                         ~~~
 1438 |         }
 1439 |         IfGuardExpr {