Reconstruct negative literal in parser

[inmaldrerah]

Before this PR, negative number literals are constructed as UnaryExpr(SUB, LitConstExpr(...)). However, the std.ast library does that differently:

import std.ast.*
import std.reflect.*

main() {
  println(quote(-1))
  println(ClassTypeInfo.of(parseExpr(quote(-1))))
  println((parseExpr(quote(-1)) as LitConstExpr).getOrThrow().literal.value)
}

prints:

- 1
std.ast.LitConstExpr
-1

which means std.ast directly constructs LitConstExpr with token -1 from tokens - and 1. This PR supports such reconstruction.