Test Function on custom TsxProvider`s

[benni-tec]

What could be improved

Instead of checking by filename TsxProvider (or a new base class) could provide a function to check if a source can be resolved by the Provider.

Why should this be improved

As I have mentioned in #69 (nice) I am currently working on a CLI to optimize tmx files. For this I resolve tsx files relative to the respective tmx file. Currently this has to be done by parsing to XML beforehand and resolving the tsx locations and then generating a TsxProvider for each. This seems rather inefficient

Any risks?

I believe that this carries minmal risk, since the current functionality could be easily kept as a implementation of a new TsxProviderBase. Therefore no public contracts, naming or functionality would need to change, while greatly extending the capability of custom TsxProvider`s.

More information

I would gladly prepare a PR for this and improving documentation both in the README.md (which currently is wrong) and in code to explain this behaviour! I have already implemented this in the "#70-tsx-provider" branch of my fork.

[benni-tec]

I was also curious which purpouse the static parse(String key) method on TsxProvider servers, since static methods can not be extended in superclasses or extensions in dart as far as I know. This confused me in the beginning and since the method is ever only able to throw an exception I think it could just be removed?