mesonlib: Fix typing of substitute_values

This is really, really, annoying. What we really want is (psuedocode):
```python
class SubValues(TypedDict[str, str], total=False):

    @INPUT@: T.List[str]
    @OUTPUT@: T.List[str]
```

Which would specifiy that `@INPUT@` and `@OUTPUT@` *may* be present and
if they are, then they are lists. There may be additional keys, which
have a single string as a value. Of course there is currently no way to
do that with typing, so we have to instead take a union type and then
use asserts to help the type checker unerstand this.

More info: https://github.com/python/mypy/issues/4617

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