Add mise.gitignore

[scop]

Reasons for making this change:

I'm a mise user. Local mise configs should not be checked into git.

The upstream project spells "mise" consistently in lowercase, therefore I opted to add the file in all lowercase, too.

Links to documentation supporting these rule changes:

See comments in the added .gitignore

If this is a new template:

• Link to application or project’s homepage: https://mise.jdx.dev/