fix: stop lexing negative numbers as single tokens to fix parsing of subtraction

Previously, the lexer greedily consumed a `-` followed by a digit as a single negative number token, causing "1 -1" to be lexed as two number tokens instead of a minus operator and a positive literal. This broke parsing of subtraction expressions without spaces.

Now `-` always emits a TOKEN_MINUS, and the parser handles unary negation. This is a breaking change: `-16.sqrt` is now parsed as `-(16.sqrt)` instead of `(-16).sqrt`, requiring parentheses for negative method receivers. All number method tests (abs, ceil, floor, sqrt, toString) are updated to use explicit parentheses for negative receivers.

test/grammar.wren

@@ -25,6 +25,12 @@ IO.print(false == 1 >= 2) // expect: true
 // Unary - has lower precedence than ..
 IO.print(-"abc".count) // expect: -3
 
+// 1 - 1 is not space-sensitive.
+IO.print(1 - 1) // expect: 0
+IO.print(1 -1)  // expect: 0
+IO.print(1- 1)  // expect: 0
+IO.print(1-1)   // expect: 0
+
 // TODO: %, associativity.
 
 // Using () for grouping.