Move osmomath into its own go.mod (backport #3771)

[mergify]

This is an automatic backport of pull request #3771 done by Mergify.
Cherry-pick of 079702e has failed:

On branch mergify/bp/v13.x/pr-3771
Your branch is up to date with 'origin/v13.x'.

You are currently cherry-picking commit 079702ea.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   Makefile
	modified:   app/apptesting/gamm.go
	new file:   go.work
	new file:   go.work.sum
	new file:   osmomath/go.mod
	new file:   osmomath/go.sum
	modified:   x/gamm/pool-models/balancer/amm.go
	modified:   x/gamm/pool-models/balancer/pool_test.go
	modified:   x/gamm/pool-models/balancer/util_test.go
	modified:   x/gamm/pool-models/internal/cfmm_common/lp.go
	modified:   x/gamm/pool-models/internal/test_helpers/test_helpers.go
	modified:   x/gamm/pool-models/stableswap/amm.go
	modified:   x/gamm/pool-models/stableswap/amm_bench_test.go
	modified:   x/gamm/pool-models/stableswap/amm_test.go
	modified:   x/gamm/pool-models/stableswap/pool.go
	modified:   x/twap/logic_test.go

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both modified:   go.mod
	both modified:   go.sum
	both modified:   osmomath/decimal_test.go
	deleted by us:   osmomath/exp2_test.go
	both modified:   x/gamm/keeper/pool_service.go
	both modified:   x/gamm/pool-models/stableswap/pool_test.go
	both modified:   x/twap/logic.go
	deleted by us:   x/twap/strategy.go
	deleted by us:   x/twap/strategy_test.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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