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/sqrt.wren

@@ -1,10 +1,10 @@
 IO.print(4.sqrt)        // expect: 2
 IO.print(1000000.sqrt)  // expect: 1000
 IO.print(1.sqrt)        // expect: 1
-IO.print(-0.sqrt)       // expect: -0
+IO.print((-0).sqrt)     // expect: -0
 IO.print(0.sqrt)        // expect: 0
 IO.print(2.sqrt)        // expect: 1.4142135623731
 
-IO.print(-4.sqrt.isNan) // expect: true
+IO.print((-4).sqrt.isNan) // expect: true
 
 // TODO: Tests for sin and cos.