feat: Changes to prop close-icon

[leobia]

Description

Improved slot close-icon to be more customizable. Now it can be used to introduce new buttons.

Changes

• Removed v-if/v-else on slot close-icon and introduced fallback content. Basically now close-icon slot is always used, but when it is not defined the old x (the svg that was on the v-else) is used
• Introduced new prop tabCloseWidth the default value is 16 (as it was in the css class)
• Updated readme for slot close-icon
• Created new example for tabs with two buttons

Tests done

• [ x ] Test default slot on other examples
• [ x ] Test old close examples
• [ x ] Test example two buttons

[viewweiwu]

Thank you for the pr, I will confirm and release the version