fix(examples): use go.work instead of replace directive

[phlx0]

The replace directive in examples/go.mod breaks go run when fetching from the module proxy:

$ go run charm.land/bubbletea/examples/doom-fire@latest
go: charm.land/bubbletea/examples/doom-fire@latest: The go.mod file for the module
providing named packages contains one or more replace directives. It must not contain
directives that would cause it to be interpreted differently than if it were the main module.

Dropped the replace and added go.work.example instead. Copy it to go.work for local dev — same effect, doesn't break remote runs.

Also added go.work and go.work.sum to .gitignore.

Closes #1681