Add option to disable auto package insert (and some subfiles support)

[PHPirates]

Fixes #1101

This PR

• Adds a global setting to disable auto insertion of package requirements (and also disables LatexMissingImportInspection)
• The root file when using the subfiles package is now correct, so package imports will be added to the correct preamble and imports will be resolved correctly (try using autocomplete for something in section1.tex)
• Added the subfile command
• Do not add the class in the list of included packages, otherwise it thinks that subfiles is included twice.

https://github.com/Hannah-Sten/TeXiFy-IDEA/wiki/Automatic-package-importing

\documentclass{article}
\usepackage{amsmath}
\usepackage{subfiles}
\begin{document}
    \section{One}\label{sec:one}
    \subfile{section1}
\end{document}

\documentclass[main.tex]{subfiles}

\begin{document}

    \begin{align*}
        \alpha
    \end{align*}

\end{document}