Handle the `xrm_Char` case in the (rasi theme) config file parser.  This
should properly handle configuration like

```
matching-negate-char: "\0";
```

and

```
matching-negate-char: "-";
```

`mem` shouldn't ever be set when `XrmOption` is `type` `xrm_Char`.
Therefore, there is no need to check it and free it.  Remove that logic.

...and missing period.

Add a `CHAR` token that takes things of the form `'<char>'` or some
specific backslash escape sequences like `'\''` and `'\0'`.  For now,
save it as a `T_STRING`.

Instead of using strings for property elements of type char, use
characters, which were recently added to the grammar.