[parser] Figure out how to do the exception handling nicely and consistently

[MarcellPerger1]

We need to figure out how to do the exception handling nicely and consistently.

Originally posted by @MarcellPerger1 in #2 (comment)

[MarcellPerger1]

I definitely don't like the status quo of being completely inconsistent. I also don't like keeping a returns @Nullable and a throws ExprParseExcption variant of all functions - much more maintenance, might forget to add it for a new parse___ function.

So we have 2+1 options:

1. Make everything return @Nullable MathSymbol or Optional<MathSymbol>
2. Make everything use throws ExprParseException - subclass from RuntimeException or Exception?
3. Make everything return a custom Rust-style Result<V, E> (yes it is possible in Java)

[MarcellPerger1]

	@Nullable / Optional	throws ExprParseException	Result<V, E>
BNF Unions in the code	✅ Easy	❌ Annoying to do big try/catch	✅✅ Easy, custom methods
Error messages	❌ NO	✅ Yes	✅ Yes
Error propagation	⚠️ Manual	✅ Automatic	☑️ Probably: then, etc.
Stack trace	❌ NO	✅ Automatic	☑️ Can store exception w/ stack trace in it
Syntax	☑️ Good, but manual	✅ Nice, automatic, cannot forget, first-class	⚠️ A bit annoying
Other		❌ Implicit, easy to forget about	✅ Explicit; ✅ elegant inline chaining methods; ❌ Annoying to do in loops where exceptions would be automatic; ✅ Easy to convert to others

[MarcellPerger1]

So use CHECKED throws ExprParseException as that is explicit, can use Result internally for BNF unions.