Add Mansi layout

[tenextractor]

See the contributing section in the readme for more detailed guideline: https://github.com/Helium314/HeliBoard?tab=readme-ov-file#guidelines
tl;dr (you should still read the full list though):

• make sure it's wanted
• a single thing only
• describe it properly
• re-use existing mechanisms / code
• low performance impact
• make it a draft if you still want to work on it
• don't do translation PRs

Further

• When the PR contains "fixes" , the related issue will be linked and automatically closed if the PR is merged (also works for other words like "fix", "resolve", "resolves", "closes", ...)
• If you add a keyboard layout, make sure you have read https://github.com/Helium314/HeliBoard/blob/main/layouts.md#adding-new-layouts--languages
• Please avoid force-pushing when doing changes. This way it's not possible which parts have changed since the previous state.

Add (Northern, but can also be used to type Western) Mansi layout. Partial fix for #762
There is an issue, the language name is shown as 'mns' in the language selection screen and the spacebar, despite adding a string for subtype_mns.

Is there any way to fix this? The Kaitag language option shows up just fine.

[tenextractor]

[Helium314]

The Kaitag language option shows up just fine

See the Kaitag PR: https://github.com/Helium314/HeliBoard/pull/519/files#diff-ceae6d63409703480a59c2cc7a80ba68782ef01acc56df9aca17be9d617bf68b
It should work once you add a string resource for subtype_mns.

[tenextractor]

Thanks for the help, it works now