Support for legacy .strings and .stringsdict file formats

[liamnichols]

• Closes Support for reading legacy strings files and providing fallback strings #18

[liamnichols]

Detecting the development language is not so straightforward.. Need to ignore others.

[liamnichols]

It seems that a SPM environment will properly propagate the defaultLocalization into the DEVELOPMENT_LANGUAGE build setting, which can be read by the CLI from the environment variables.

It does however prevent us from filtering the build inputs to ignore other languages... An acceptable tradeoff maybe?

[liamnichols]

Introduced an InputParser that takes the DEVELOPMENT_LANGUAGE into account when reading .strings and .stringsdict files from the inputs. This ensures that it doesn't try to load other languages.

The downside is that all .strings/.stringsdict files are considered inputs to the plugin, which means that the plugin will re-run when translations change even though they won't have an input on the generated source, but that is considered a reasonable tradeoff to get this legacy support out there.