Split StrataMain.lean into importable library and thin executable root (#1179)

Fixes #1163

Moves all library code from `StrataMain.lean` into a new `StrataMainLib`
module, leaving `StrataMain.lean` as a thin executable entry point
(`import StrataMainLib`). This makes the library importable from tests
and other tools without pulling in the executable entry point.

Author: MikaelMayer