Add template suggestions for bundle and view mode

[tanc]

Adds templates suggestions for the bundle and view mode, will look like this:

<!-- THEME DEBUG -->
<!-- THEME HOOK: 'localgov_geo' -->
<!-- FILE NAME SUGGESTIONS:
   * localgov-geo--mybundle--myviewmode.html.twig
   * localgov-geo--mybundle.html.twig
   * localgov-geo--myviewmode.html.twig
   x localgov-geo.html.twig
-->

[tanc]

Fixes #13

[finnlewis]

Trivial enough clean change, tests pass.
Looks good to me!

[finnlewis]

CC @cjstevens78 @Adnan-cds @andybroomfield just FYI really, will probably be useful!

[tanc]

Just had to add a fix as the suggestions weren't actually working. Everything has to be underscores which then get converted to dashes. So while it looked like suggestions were working in the twig debug output they actually weren't getting picked up if you had a file named like one of the suggestions. A classic Drupal WTF!

For example:
$suggestions[] = 'localgov-geo__' . $sanitized_view_mode;
has to be:
$suggestions[] = 'localgov_geo__' . $sanitized_view_mode;

[Adnan-cds]

Thanks Tanc :)