Create the internal/tui package skeleton which wraps BubbleTea Implementation

[baync180705]

Initialize the internal package that will hold the Bubble Tea logic.

• Create directory internal/tui.
• Create internal/tui/model.go (defines the tea.Model).
• Create internal/tui/styles.go (defines Lip Gloss styles).
• Implement a basic InitialModel() function that returns a tea.Model.
• Implement the required Bubble Tea methods: Init, Update, and View.