fix: export MixedSchema to fix ts with --declarations (#1204)
fix: export MixedSchema to fix ts with --declarations (#1204)

Fixes ts error `Exported variable 'Schema' has or is using name 'MixedSchema' from external module "x/node_modules/yup/lib/mixed" but cannot be named.` with --declarations builds.

The MixedSchema class needs to be exported so it can be used in declaration files.
jquense/yup,jquense/yup