Build LDC LTS as a separate flatpak?

Currently, we use old LDC LTS for bootstrapping. We could submit it as a separate SDK extension and still use it for building LDC. This would drastically reduce build times for LDC. Also maybe someone can make use of LDC LTS for, well, something.

Start by inspecting how the current old LDC LTS is used for bootstrapping and how this repository defines its Flatpak SDK extension. Clarify whether a separate LDC LTS extension is wanted, then establish measurable build-time improvements and the expected way other builds can consume it before implementation.

Written by the indexing model from the issue text.