Add dark-L vowels to EnXSampaPhonemizer.cs

[HitCoder9768]

for reclists such as my own using phonemes such as O5 for "cold"

[oxygen-dioxide]

The GitHub action tests failed. Please ensure that the unit tests passed. You can use dotnet test command to run the tests locally. If your code changes the behaviour of the phonemizer, also change them in the unit tests.

[lottev1991]

(Apologies, I sent this on Discord too but it's kinda important) I also noticed that you've defined the Ol vowel twice; it was already defined, and defining it twice will break the code and the phonemizer will not work (see oxygen's message; I think this is why the tests might be failing). Please fix that, thanks in advance.