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/number/abs.wren

@@ -1,6 +1,6 @@
-IO.print(123.abs)    // expect: 123
-IO.print(-123.abs)   // expect: 123
-IO.print(0.abs)      // expect: 0
-IO.print(-0.abs)     // expect: 0
-IO.print(-0.12.abs)  // expect: 0.12
-IO.print(12.34.abs)  // expect: 12.34
+IO.print(123.abs)      // expect: 123
+IO.print((-123).abs)   // expect: 123
+IO.print(0.abs)        // expect: 0
+IO.print((-0).abs)     // expect: 0
+IO.print((-0.12).abs)  // expect: 0.12
+IO.print(12.34.abs)    // expect: 12.34