Add supports for `System.Text.Rune` API

[MangelMaxime]

Description

For a library I am working on, I will need the System.Text.Rune API.

At the moment here is the API I am using:

rune.* are instance methods / properties

• Rune.GetRuneAt
• rune.ToString()
• rune.Utf16SequenceLength
• rune.Value
• c.EnumerateRunes() => I could probably use Rune.GetRuneAt with a while loop to mimic this API

I think the most difficult part will be to get the rune.Value because we need to find how to convert a character/string char into this value.

Once this is done, adding the different properties / static member should not be too difficult. For JavaScript, this file can be a source of inspiration on how to deal with unicode characters especially how to detect the ones using 2 bytes in UTF 16.

// Detect 2 bytes characters
(string.charCodeAt(offset) & 0xF800) === 0xD800

I am creating this issue just to keep track of this future work.