Change default multiplication behavior

[ZibingZhang]

Overview

By default, multiplication should have a \cdot.

Details

Based on this discussion, it's clear that not having \cdot is the exception rather than the rule.

As a precursor to work implementing the exception behavior, we should first move the default behavior to include a \cdot, in order to make the future PR easier to follow. We basically split the that issue into two

1. first change default behavior
2. TODO: implement exceptions

References

https://github.com/google/latexify_py/issues/89

Blocked by

None