perf: Minor optimizations

[hangy]

Some minor performance optimizations I stumbled upon while having a look at #341:

1. FallbackLanguagesCollection
   1. Avoid dictionary lookup for the "default" fallback case
   2. Avoid ContainsKey/[key] lookup by shortcutting with TryGetValue
2. JsonConverter
   1. Replaced ICollection<string> with List<string> to avoid unnecessary virtual method calls
   2. Use char instead of string
   3. Optimize some string/collection operations with range operator and Span<T>
3. LocalizationProvider
   1. Use source-generated compiled regex to detect placeholders
   2. Use a token-based approach to replace placeholders to avoid regex allocations

[stefanolsen]

@hangy Can you also replace new CultureInfo(language) with CultureInfo.GetCultureInfo(language) in FallbackLanguagesCollection.cs, since you are already working on that file?

[hangy]

@hangy Can you also replace new CultureInfo(language) with CultureInfo.GetCultureInfo(language) in FallbackLanguagesCollection.cs, since you are already working on that file?

I can't find it there. You're talking about this file, correct?

[stefanolsen]

@hangy My bad. I was referring to this.

[valdisiljuconoks]

@stefanolsen I'll finish CultureInfo.Get(...) in v9 branch