fix(vlistgroup): fixed icon not displaying using kebab-case

[louisdelphie]

added prepend-icon and append-icon to slot.

fix #8676

Description

Added !this.$slots['append-icon']/!this.$slots['prepend-icon'] to the if condition.
Added this.$slots['append-icon']/this.$slots['prepend-icon'] to the $createElement function
call in genAppendIcon/genPrependIcon functions.
fixes #8676

Motivation and Context

#8676 #8676

How Has This Been Tested?

I tested visually

Markup:

// Paste your FULL Playground.vue here

{{ iconName }} Users <script> export default { data: () => ({ iconName: 'people_outline', admins: [ ['Management', 'people_outline'], ['Settings', 'settings'], ], cruds: [ ['Create', 'add'], ['Read', 'insert_drive_file'], ['Update', 'update'], ['Delete', 'delete'], ], }), } </script>

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any features but makes things better)

Checklist:

• The PR title is no longer than 64 characters.
• The PR is submitted to the correct branch (master for bug fixes and documentation updates, dev for new features and backwards compatible changes and next for non-backwards compatible changes).
• My code follows the code style of this project.
• I've added relevant changes to the documentation (applies to new features and breaking changes in core library)