Make TM2 independent from GnoVM

[moul]

Continues #585

• Migrate tm2/pkg/sdk/vm and pkg/crypto/keys/client parts depending on gnovm either in gnovm or gno.land. @piux2
• Setup go mod workspaces.
• Create standalone tm2 demo chain + client (done in chore: post-split cleanups tendermint/tendermint2#1). @moul
• Setup vanity URLs for tm2 and "the rest". @moul
• Improve CI, ensuring tm2 won't depend on nothing "gno*", i.e., grep gnovm from dedicated go.mod.