Make axis environment a pgfscope

[schtandard]

Rather than just a TeX group. This prevents changes to global variables like \pgflinewidth from leaking out of the axis environment.

Currently, an axis environment changes \pgflinewidth (and possibly other values) outside of the environment, leading to unexpected results with code that relies on that value being correct. For example, circuitikz uses this length to determine the component stroke width. After an axis (right resistor), this does not work correctly.

\documentclass{article}

\usepackage{tikz}
\usepackage{circuitikz}
\usepackage{pgfplots}
\pgfplotsset{compat=1.18}

\begin{document}

\begin{tikzpicture} [line width=2pt]
  \showthe\pgflinewidth
  \draw (-2,0) to [R] ++(2,0);
  \begin{axis} [anchor=north, yshift=-1cm]
    \addplot+ {x^2};
  \end{axis}
  \showthe\pgflinewidth
  \draw (0,0) to [R] ++(2,0);
\end{tikzpicture}

\end{document}

Using a pgfscope rather than a TeX group solves this problem.