Add electric indent for preproc directives

It looks like there are few if no other cases where we get this
particular error in the AST than with insertion of '#'.

Consider:

```
int
main()
{
  |
}
```

If we only add '#' the AST yields (ERROR (ERROR)), which we can
exploit to hook onto electric indent.  The end result should be the
hashtag anchored to column 0.

* lisp/progmodes/c-ts-mode.el (c-ts-mode--indent-styles): New rule.
(c-ts-base-mode): Add # to electric-indent-chars.

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