Default to SVG output in VSCode & Atom

Currently, svg output is disabled by default because it is really slow in IJulia, something I have tested for myself. However, displaying svgs in e.g. VSCode works well, and gives the user sharper plots as a result.

Could we switch the default in this package to display svg, and either have IJulia itself disable the svg output, or PyPlot detect that it is running within an IJulia kernel?

首先跟踪 PyPlot 的默认 SVG 设置以及 IJulia 内核如何处理输出，然后比较 VSCode 和 Atom 中的预期行为。当 SVG 成为受支持编辑器使用时的默认格式，同时 IJulia 避免报告的性能问题，并且该行为在相关测试存在时由这些测试覆盖，即视为完成。

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