Finding polynomial roots is limited to only the first 4 values of the polynomial

**Describe the bug**
When using findRoot, it is impossible to find the root of a polynomial with more than 4 elements, so (ax^3 + bx^2 + cx + d).

**To Reproduce**
Steps to reproduce the behavior:
1. Use the function on a polynomial with more than 4 coefficients/numbers (t.polynomial(1, 6, -8, -6))

**Expected behavior**
The roots should be gotten.

**Screenshots / Output**
the output is simply undefined or an empty array

**NodeJS (please complete the following information):**
- OS: [e.g. macOS 10.14] Linux ( Ubuntu 20.04 )
- Node version [e.g. v8.11.3] Node 18
- Version [e.g. v1.0] latest (1.2.0)

Start by reproducing the issue with Node 18 and TheoremJS 1.2.0 using findRoot and t.polynomial(1, 6, -8, -6). Trace the findRoot entry point and polynomial handling until the coefficient limit is identified; done means roots are returned for polynomials with more than four coefficients instead of undefined or an empty array.

Written by the indexing model from the issue text.