simplifying linear expressions

**Bug Description**
mathJS fails to simplify simple linear expressions

**To Reproduce**
for example:
`simplify('7*(-t-13) -4 -12t')`
returns:
`7 * (-t - 13) - 12 * t - 4`
instead of:
`-19 * t - 95`

Start by reproducing the simplify('7*(-t-13) -4 -12t') entry point and inspect how the expression is transformed. Done means simplify combines the terms and returns -19 * t - 95 for this example, with regression coverage for the reported behavior.

Written by the indexing model from the issue text.