[FEATURE] Supporting Matrix Notation

Hi @Kolaru ,

I was tinkering with MathTeXEngine and was trying to render a matrix.
Using `LaTeXStrings.jl`, I wrote the following snippet:

```julia
latex_string = L"""
$\begin{matrix}
1 & 2 & 3\\
a & b & c
\end{matrix}$
"""
```

And when I tried rendering it like so:

```julia
generate_tex_elements(latex_string)
```

I got the following error:

```julia
ERROR: TeXParseError: unexpected error while parsing
at position 7 (string index 7)
\begin{matrix}
1 & 2 & 3\\
a & b & c
\end{matrix}
^
Stack before
[1] TeXExpr :expr

Stacktrace:
[1] texparse(data::String; showdebug::Bool)
@ MathTeXEngine ~/.julia/packages/MathTeXEngine/zH12o/src/parser/parser.jl:285
[2] texparse
@ ~/.julia/packages/MathTeXEngine/zH12o/src/parser/parser.jl:271 [inlined]
[3] _broadcast_getindex_evalf
@ ./broadcast.jl:670 [inlined]
[4] _broadcast_getindex
@ ./broadcast.jl:643 [inlined]
[5] _getindex
@ ./broadcast.jl:666 [inlined]
[6] _broadcast_getindex
@ ./broadcast.jl:642 [inlined]
[7] getindex
@ ./broadcast.jl:597 [inlined]
[8] copy
@ ./broadcast.jl:899 [inlined]
[9] materialize
@ ./broadcast.jl:860 [inlined]
[10] generate_tex_elements(str::LaTeXString, font_family::MathTeXEngine.FontFamily)
@ MathTeXEngine ~/.julia/packages/MathTeXEngine/zH12o/src/engine/layout.jl:295
[11] generate_tex_elements(str::LaTeXString)
@ MathTeXEngine ~/.julia/packages/MathTeXEngine/zH12o/src/engine/layout.jl:289
[12] top-level scope
@ REPL[12]:1

caused by: TeXParseError: unsupported command \begin
at position 7 (string index 7)
\begin{matrix}
1 & 2 & 3\\
a & b & c
\end{matrix}
^
Stack before
[1] TeXExpr :expr

Stacktrace:
[1] _end_command_builder!(stack::DataStructures.Stack{Any}, p::Int64, data::String)
@ MathTeXEngine ~/.julia/packages/MathTeXEngine/zH12o/src/parser/parser.jl:233
[2] macro expansion
@ ~/.julia/packages/MathTeXEngine/zH12o/src/parser/parser.jl:256 [inlined]
[3] macro expansion
@ ~/.julia/packages/Automa/1KOLQ/src/codegen.jl:149 [inlined]
[4] texparse(data::String; showdebug::Bool)
@ MathTeXEngine ~/.julia/packages/MathTeXEngine/zH12o/src/parser/parser.jl:283
[5] texparse
@ ~/.julia/packages/MathTeXEngine/zH12o/src/parser/parser.jl:271 [inlined]
[6] _broadcast_getindex_evalf
@ ./broadcast.jl:670 [inlined]
[7] _broadcast_getindex
@ ./broadcast.jl:643 [inlined]
[8] _getindex
@ ./broadcast.jl:666 [inlined]
[9] _broadcast_getindex
@ ./broadcast.jl:642 [inlined]
[10] getindex
@ ./broadcast.jl:597 [inlined]
[11] copy
@ ./broadcast.jl:899 [inlined]
[12] materialize
@ ./broadcast.jl:860 [inlined]
[13] generate_tex_elements(str::LaTeXString, font_family::MathTeXEngine.FontFamily)
@ MathTeXEngine ~/.julia/packages/MathTeXEngine/zH12o/src/engine/layout.jl:295
[14] generate_tex_elements(str::LaTeXString)
@ MathTeXEngine ~/.julia/packages/MathTeXEngine/zH12o/src/engine/layout.jl:289
[15] top-level scope
@ REPL[12]:1
```

I saw that this issue was somewhat similar to supporting `\begin{align}` statements in #25 .
So, out of curiosity, how difficult would it be to create support for rendering matrices in `MathTeXEngine`?
Is this something that is out of the purview of the engine or would it be doable?