usageClassName not applied correctly

[filoxo]

Expected behavior

When usageClassName is provided, the output svg spritesheet should use it to generate the target styles.

Current behavior

When usageClassName is provided, the output svg spritesheet does not use it but instead uses the default value.

Steps to reproduce

Using the following config:

const spritesheet = await mixer('src/svg/*.svg', 
  {
    spriteType: 'stack', 
    spriteConfig: {
      usages: false,
      usageClassName: 'icon-sprite',
    },
  }
)

My spritesheet is emitting the following (formatted for clarity):

<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
  <defs>
    <style>.sprite-symbol-usage {display: none;}
.sprite-symbol-usage:target {display: inline;}</style>
  </defs>
  <svg viewBox="0 0 24 24" id="icon-id" class="icon-sprite"><path d="..." /></svg>
</svg>

Note that the style definition uses .sprite-symbol-usage but the class applied to the inner svg element is icon-spite as configured.

The traced the issue back to this config setup: https://github.com/JetBrains/svg-mixer/blob/master/packages/svg-mixer/lib/stack-sprite.js#L16-L22 I'm not an expert with classes but I think what is happening is that the getter's this will refer to the original object that encloses it, and not a reference to the class after merge. I have a fix but ultimately couldn't get tests working so will submit a PR and hope CI can validate it.