[skip ci]

…1977)

Previously, when a `cz_customize` user supplied a custom `bump_map`
but no explicit `bump_map_major_version_zero`, `cz bump` with
`major_version_zero = true` silently fell back to
`defaults.BUMP_MAP_MAJOR_VERSION_ZERO` -- a totally unrelated mapping
that ignored the user's bump rules. The user's intent (e.g. `feat` =
PATCH while major version is 0.x) was overridden.

When the user provides `bump_map` but not
`bump_map_major_version_zero`, derive the latter from the former by
demoting any `MAJOR` rule to `MINOR` -- the only difference in the
default mapping pair. Users who want a custom split between the two
maps can still set `bump_map_major_version_zero` explicitly.

Closes #1728

Co-authored-by: Copilot <[email protected]>