[TwigBundle] Add loader priority

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   |  
| Fixed tickets | 
| License       | MIT
| Doc PR        |  

Add the ability to specify a priority to the tag of `twig.loader` services.

eg:
```
        <service id="twig.loader.filesystem" class="%twig.loader.filesystem.class%" public="false">
            <argument type="service" id="templating.locator" />
            <argument type="service" id="templating.name_parser" />
            <tag name="twig.loader" priority="100"/>
        </service>
```
is ksort correct here?  Higher priority needs to be first.