Ignore \begin and \end commands in \newcommand

[PHPirates]

Fix #3101

Summary of additions and changes

• Lex \begin and \end in \newcommand and \providecommand as regular commands, as they are often unmatched

How to test this pull request

\newcommand{\tableMod}[0]{
    \end{multicols}
    \insertedObject
    \begin{multicols}{2}
}

• Updated the documentation, or no update necessary
• Added tests, or no tests necessary
• Parser issue: \newcommand{\cmd}{$x$}