Matrix environment support

This is a successor task for #100.

In #155, we've successfuly implemented commands such as `\matrix`, `\cases` and `\pmatrix`.

Also, there's an internal support for environments. Let's make it official and add all the matrix environments from [that article](https://math-linux.com/latex-26/faq/latex-faq/article/how-to-write-matrices-in-latex-matrix-pmatrix-bmatrix-vmatrix-vmatrix):

- [ ] `\begin{matrix}`,
- [x] `\begin{pmatrix}`,
- [ ] `\begin{vmatrix}`,
- [ ] `\begin{Vmatrix}`

should all work.