SVG Twig Extension is in the core (previously, it was a dedicated extension pushword/svg).
By default, it's use FontAwesome. You just need to use twig svg function :
{{ svg('surprise') }}
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By default, this extension load FontAwesome's icons.
You can choose to use your custom SVG icons by specifying the svg_dir in configuration. Default paths include:
%kernel.project_dir%/templates/icons%kernel.project_dir%/public/bundles/pushwordcore