fix(icon-button): fix to properly default `theme` to "default" to allow for "primary" to be used like other themes

[DRiFTy17]

PR Checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added/updated: Y
• Docs have been added/updated: Y
• Does this PR introduce a breaking change? N
• I have linked any related GitHub issues to be closed when this PR is merged? N

Describe the new behavior?

The <forge-icon-button> previously incorrectly set its default theme state to "primary" which doesn't allow for the theme to be truly using the "primary" theme color for the icon like the other built-in theme colors do. This was done because by default icon buttons will use currentColor to inherit the color for the icon, but the state layer and focus indicator both use the primary theme.

This change will reduce confusion by setting the default state to "default" instead, which enable the "primary" theme color to naturally just work as expected to fully style it correctly.

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:rocket: PR was released in v3.1.0 :rocket: