Nested type names were being highlighted as if they were function names.

Now a phrase like `Spec.Process.run` is highlighted properly as:
- `Name.Class`
- `Punctuation`
- `Name.Class`
- `Punctuation`
- `Name.Function`

Instead of wrongly highlighted as it was before this commit:
- `Name.Class`
- `Punctuation`
- `Name.Function`
- `Punctuation`
- `Name.Function`

Now the Savi lexer can tokenize string interpolation inside strings,
which is a new feature added to Savi since the last time this
lexer was updated.

Prior to this change, every underscore/private identifier
in Savi code was being highlighted as if it were a class name.
After this change, only those whose first letter is uppercase
will be highlighted as such, which is the correct behavior.

The previous example shows the old usage of the testing framework.
Now the example reflects the current usage/API of that framework.

* Add lexer for the Savi language.

* Updates based on review comments.

* Prefer Text.Whitespace token over Text token for whitespace.

* Updates to Savi lexer based on changes to Rouge Savi lexer.

* Add versionadded identifier to Savi lexer.