Resolve "Javascript ffi can fail when using jsForceFetch with a very small 64 bit integer and tracing the result."

What does this MR do?

Fixes #126 (closed). The toString instance for JSInt incorrectly assumed that the absolute value of the argument was positive, while abs SmallestInt = SmallestInt.

Changes to public APIs

None.

Author's checklist (required)

• If bugs have been solved, tests have been added (guidelines)
• A changelog entry has been added (guidelines)
• Newly added code is documented (guidelines)
• Newly added code has a style consistent with the rest of the repository
• Intermediate commits compile (use git rebase -i main if not)

Related issues

Closes #126 (closed)