feat(thundermail-mvp): set the thundermail feature foundation

[rafaeltonholo]

This pull request includes the changes already present on feature/thundermail, but targeting the MVP instead of the full feature.

From #10913:

• Add contentPadding parameter to ButtonOutlined
• Add leading icon to button filled
• Add customizable colours and shapes to design system buttons

From #10914:

• Add stroke support to iconPath in IconBuilder
• Add FilledThundermail icon to design system
• Add qr-code icon

From #10915:

• Add the scaffold project for the Thundermail feature
• Wire the Thundermail :k9app and :thunderbird dedicated implementation with their respective apps.

From #10931:

• Migrate Thundermail modules to full Android modules to avoid unnecessary workarounds, as part of our design system is not yet kmp

[wmontwe]

LGTM, I think we could patch the design later once clarified.

[wmontwe]

I noticed that some of the spacing and sizing values don’t align with the Android 4dp grid (e.g. the 18dp icons). It looks like parts of the design might be based on a 3px desktop grid.

I’ve reached out to design to verify whether this was intentional. The 18dp icons stand out compared to our standard 24dp icons and are also not cleanly dividable by 4.

[rafaeltonholo]

Thanks for the review, @wmontwe! I've updated the ButtonFilled API to match with the ButtonOutlined. I also fixed a bug on ButtonOutlined, which, when it was disabled, the border was not updating its colour, and the text was transparent.

[wmontwe]

LGTM