fix(coachmark): make step count separator configurable for localization

[tmchow]

Description

The Coachmark component's step count (e.g., "1 of 3") had the "of" string hardcoded, preventing localization. This PR adds a step-count-of attribute (property: stepCountOf) that defaults to "of" and is used in the step count template.

Clients can now set step-count-of="von" for German or step-count-of="de" for Spanish without needing to use the step-count slot override. The existing slot override still works for full customization.

<sp-coachmark step-count-of="von" current-step="1" total-steps="3">
  ...
</sp-coachmark>

Fixes #4780

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This contribution was developed with AI assistance (Claude Code).

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