tweak(csharp): add open brace to electric chars

[Stewmath]

With this commit, the electric characters list is now the same in csharp-mode as in cc-mode.

Before this commit, typing "if (true) RET { RET" would produce the following:

if (true)
    {

    }

After this commit, it produces the following:

if (true)
{

}

---

• I searched the issue tracker and this hasn't been PRed before.
• My changes are not on the do-not-PR list for this project.
• My commits conform to Doom's git conventions.

[doom-bot]

(Note: this is @hlissner; I forgot to swap accounts)

This problem with this tweak is that it triggers a reindent of the line when typing a { for folks using the inline style, i.e.

if (true) {

}

I'll explore some alternatives first before revisiting this PR.