String positions and indices are clamped before they reach a pointer or a count

[ASDAlexander77]

Summary

String positions and indices are clamped before they become a pointer into the string or a byte count.

• slice overflow (String.slice(start, end) with end < start overflows the result buffer #27): an end at or before the start gave a negative count, which resize and memcpy took as a huge size. slice now returns "", as in JavaScript. It does not swap the way substring does.
• Unsigned parameters: the indexEnd of slice/substring and the position of endsWith/lastIndexOf defaulted to this.length and so took its unsigned type, although lib.d.ts declares int. Every < 0 check was dead, and "abcdef".slice(1, -3) returned "bcdef". They are annotated int now.
• Clamping: positions are clamped to [0, length]. Before, startsWith, includes and indexOf read before the string for a negative position, and endsWith and lastIndexOf read after the terminator for a position past the end.
• Behaviour changes:
  • indexOf past the end returned the length for any search string. Now only "" is found there, as in JavaScript.
  • includes(null) returns false. Before, it passed null to strstr.
  • lastIndexOf(null) returns -1. Before, it returned the position.
  • Both now match indexOf.
• trim / trimEnd: they passed the last kept character as substring's exclusive end. " ab ".trim() was "a", and a string of spaces kept all but one of them.

Fixes #27

The compiler half of this pass (string concatenation without strcpy/strcat) is ASDAlexander77/TypeScriptCompiler#520. The two are independent.

Test plan

• New tests/string_bounds.ts: each case above, checked against JavaScript's results
• Release compile, Windows: gc 162/162; rc and none 161/162 (one gc-only skip)
• JIT mode, debug, Linux: CI. Locally, any JIT run against a freshly built gc DefaultLib hangs, on main too. That is being looked into separately.

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