Add ability to configure atomic saving

[StratusFearMe21]

For some file watching/hot reloading programs, the way that Helix writes files can confuse them and cause them to throw errors or corrupt their caches. One such program is the parcel bundler

https://parceljs.org/features/development/#known-issues-with-file-watching

They explicitly document that if your editor saves files using an "atomic save" or "safe write" or "backup copy" strategy, that it should be disabled. Helix currently does not allow you to disable this functionality, and adding this configuration option makes it so that that is possible.

[jedahan]

Would this support setting the flag as a program argument? Say I only want it off for editing crontab and otherwise on? Or would I maintain a secondary config file and just pass that in? Thank you for making this contribution

[StratusFearMe21]

@jedahan , my use case for this flag would be turning atomic_save off for certain projects via the .helix/config.toml file at the root of each project