Fix issue where oniguruma causes Jest tests to fail

[jkillian]

Requirements for Contributing a Bug Fix

Identify the Bug

This fixes the same issue described in #111 and in jestjs/jest#3552, that oniguruma will not work in jest when multiple tests use it.

Description of the Change

By marking length as configurable, we prevent an error from being thrown if the code is run a second time (as it will be in a jest context).

Possible Drawbacks

As far as I know, this should be harmless enough.

Verification Process

Tried out the change locally.

Release Notes

• Fixed an issue where a reimport of the library would fail in Node.js

[icecream17]

The property descriptor is equivalent to the one you get when writing:

class OnigString {
  get length() { return this.content.length }
}

Object.getOwnPropertyDescriptor(OnigString.prototype, 'length')