fix: save as new file when file already exists

[jguegel]

Type of change(s)

• Bug fix
• Feature / enhancement
• Infrastructure / tooling (CI, build, deps, tests)
• Documentation

What changed and why

• Changed so that SaveAsDialog now appends "_{index}" to filename if file already exists

Closes the following issue(s)

• Closes File Manager not offering warning before overwriting file #131

Checklist

• I read the contribution guidelines.
• I manually tested my changes on device/emulator (if applicable).
• I updated the "Unreleased" section in CHANGELOG.md (if applicable).
• I have self-reviewed my pull request (no typos, formatting errors, etc.).
• All checks are passing.

[naveensingh]

You didn't add the Save As dialog or the toast message, but this works too, so I'll take it. Thanks.