feat(voip/mumble): Add MUMBLE_DOES_CHANNEL_EXIST native

[niekschoemaker]

Goal of this PR

To add the ability to check if channels exist, which is usefull to avoid issue #2677

Additionally changed the insert call for channels to emplace (same way as done in users) and cache the widened channel name before a for loop where this was not done yet.

How is this PR achieving the goal

It adds the native MUMBLE_DOES_CHANNEL_EXIST which allows scripts to check if a mumble channel exists before executing a native on said channel ensuring that the native actually has effect.

This PR applies to the following area(s)

FiveM & RedM (voip/mumble)

Successfully tested on

Game builds: 9073

Platforms: Windows

Checklist

• Code compiles and has been tested successfully.
• Code explains itself well and/or is documented.
• My commit message explains what the changes do and what they are for.
• No extra compilation warnings are added by these changes.

Fixes issues

fixes #2677

[niekschoemaker]

Force pushed to fix commit title from feature to feat

[FabianTerhorst]

Thanks for your contribution. 👍