Added RANDOM, RANDOM_HEX snippet variables

[Krystofee]

This PR fixes #80297

I've take the issue since there was no activity. I will be happy to write tests, when I'll get approve for this functionality.

Ping @jrieken.

[msftclas]

All CLA requirements met.

[jrieken]

lgtm

[SnirBroshi]

Why is this specifically 6 random characters?
This is very arbitrary and the use cases for exactly 6-digit random decimal or hex numbers other than a hex color are non-existent.

This should be removed @jrieken.

[jrieken]

So, what is your suggestion then? Just remove? 5 numbers? 3?

[seognil]

Wow, thanks for the work
the origin request is from me

I agree with NotWearingPants
the approach is not flexible
but it's enough for my original proposal already

I thought that maybe the 'snippet system' could support inbuilt function rather than const only
or as what I imaged before: parse any custom valid JS statements, so that vscode itself would keep simple

the first case is like `color: #${RANDOM(6, hex)}` or `console.warn('tag-${RANDOM(8, 36)}')`
the customable way is like `console.warn(${Math.random().blahblah()})`

Anyway, thanks for the work~