feature: allow to customise the default values of variant properties

[carlosdevpereira]

Description

Currently the plugin automatically checks which "variant properties" that each figma asset defines and tries to convert those variant properties into props (when generating Vue components). When these props are generated the default value is always the first value that the plugin found for that variant property.

Acceptance Criteria

• Add a new "select" form control to the plugin UI that allows the user to customise the default value of each variant property that the plugin found
  • The options inside these "select boxes" should match the possible values found for the variant property

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Relates with: #4

[carlosdevpereira]

Implemented in b0d3054