Tracking issue: improved UX

[alexpovel]

See #145 : UX around the replacement action could be improved. I suspect srgn is mainly useful as a ripgrep-like search tool. Hence, replacement could become its own option, just like all other actions (currently replacement is a positional argument), without breaking too many workflows, or incurring too much "typing effort". It would break tr-like behavior:

srgn/README.md

Line 1866 in 1b27825

## Comparison with `tr`

deviating from that compatibility a slight bit, but it might be a niche use case anyway. I don't know.

Alternatively, we could use https://docs.rs/clap/latest/clap/struct.Arg.html#method.last , which would turn into usage such as:

$ echo "Hello a" | srgn a -- b
Hello b

aka -- becomes mandatory, and replacement can be no longer be accidentally inserted into surprising spots, as in #145 . That seems like a neat middle ground, but if we break backwards compatibility anyway, might as well streamline all actions to be given as options throughout (and stop treating replacement specially).