Add support for numbering level overrides

[felipeochoa]

Level overrides allow two different numbering schemes to share a single abstract numbering definition if they only differ in a few places. Beyond the usual styling properties, this allows you to override the numbering sequence, e.g., to skip a number:

1. Option 2
2. Option 3
5. [eliding option 4] Option 5

The api for this is:

const numbering = new Numbering();
const abs = numbering.createAbstractNumbering();
abs.createLevel(0, "lowerLetter", "%1)", "start").indent(1260, 540);
// ... create more levels
const num = numbering.createConcreteNumbering(abs);
num.overrideLevel(levelNum, newStart).level.indent(1500, 780);  // Can set indent, font, etc.