Spin out `five-point-zero` branch & scaffold

[petecooper]

The dev branch of https://github.com/textpattern/textpattern/ is intended for Textpattern 5.0, but it's currently using the four-point-nine theme with a tweaked manifest to give it a 5.0.0-dev version.

We need to:

• create a 5.0.x branch on https://github.com/textpattern/textpattern-default-theme/
• create a dist/five-point-zero dir + file scaffold in https://github.com/textpattern/textpattern-default-theme/ (in main and the above 5.0.x branch, presumably)
• create a five-point-zero dir + file scaffold in https://github.com/textpattern/textpattern/tree/dev/textpattern/setup/themes
• check https://github.com/textpattern/textpattern/blob/dev/.github/get-default-theme.sh is doing its thing properly

Have I missed anything?