Refactored `TexPredefinedFormulaParser` to improve readability and ascertaining more invariants

[Lehonti]

• Sealed the class (TexPredefinedFormulaParser) itself
• Fields with type IDictionary<,> never change (are only read, not written to) after the object is constructed, so I made them of type IReadOnlyDictionary<,>. All entry additions happen inside the constructor/s (I'm including the static constructor, as we will see in the next point).
• typeMappings and argValueParsers are always the same, in all instances, so I made them static and moved their construction to the static constructor. This can help us save on heap allocations.
• Sealed the private parser classes. Also, since most of them don't hold any internal state (and if the maintainer agrees) they can be turned into singletons in a future pull request.
• Nesting levels were decreased in the Parse() method through inversion of if blocks.

[ForNeVeR]

Sealed the private parser classes. Also, since most of them don't hold any internal state (and if the maintainer agrees) they can be turned into singletons in a future pull request.

Feel free to do it in a separate PR.