Rename `-s/--string-mode` to `-F/--fixed-strings`

[zx8]

I assume the majority of users who come across sd are likely also using other popular Rust-based CLI tools like fd and rg:

• sharkdp/[email protected]#L70-L75
• BurntSushi/[email protected]#L1175-L1178

For consistency with the above, it would be neat if sd's options were consistent with these to make context switching a bit easier.

[NightMachinery]

(Don't break the API, add a new option.)

[seguri]

Not only Rust-based new tools but also good old grep...

[CosmicHorrorDev]

-s, --string-mode is now -F --fixed-strings with an alias to ensure that any usages of the old flags still work