[Feat] add edit message and retry generation

[a-ghorbani]

Description

Adds a long-press menu for chat bubbles (+few other changes, see below):

For user messages:
- Copy: Copies the message to the clipboard
- Edit: Lets the user edit any previous message and restart from there.

For model responses:
- Copy: Same as above.
- Regenerate: Regenerates the response.
- Regenerate with: Regenerates the response with another available model.

Additional changes:

• Introduced new menu components: Menu, MenuItem, SubMenu
• Refactored theme colors.
• Renamed the chat Input component to ChatInput
• Updated/refactored tests and mocks affected by these changes.

Known Issue:

• Menu does not inject onSubmenuOpen and onSubmenuClose into Submenu.

Fixes #62
Fixes #61

Platform Affected

• iOS
• Android

Checklist

• Necessary comments have been made.
• I have tested this change on:
  • iOS Simulator/Device
  • Android Emulator/Device
• Unit tests and integration tests pass locally.