Enable nullable references

[perlun]

Moved to GitLab

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Nullable references (or rather, non-nullable references) is one of the cooler features in more recent versions of C# (more specifically, C# 8.0 or greater - .NET Core 3.0 and upwards) We have been a bit sloppy here, but we should enable these compiler flags to try and take advantage of this support right from start, especially since the Perlang language has strong ambitions regarding non-nullability in itself.

[perlun]

Some of this sneaked in as part of #77 being merged. I found out that it's possible to enable nullability warnings for individual files (like this). This is incredibly convenient, since it means we can gradually start moving towards more and more nullability types until the whole codebase is converted to use it.

The newly created Perlang.Tests project also has nullability support enabled project-wide. Perlang.Tests.Integration could be a reasonable next project to enable it for.

[perlun]

Not (fully) accomplished in 0.2.0, postponing to a later iteration.

[perlun]

Our current state of affairs, a few years later:

$ rg Nullable $(find -name \*.csproj)
./src/Perlang.Parser/Perlang.Parser.csproj
5:        <Nullable>disable</Nullable>

./src/Perlang.Interpreter/Perlang.Interpreter.csproj
5:        <Nullable>disable</Nullable>

./src/Perlang.Common/Perlang.Common.csproj
6:        <Nullable>disable</Nullable>

./src/Perlang.Tests/Perlang.Tests.csproj
6:        <Nullable>enable</Nullable>

I was reminded of this since I tried to enable it for Perlang.Parser now, but it's a bit of work to get it done. We should aim for doing this; it's not a huge effort and it will "feel good" to take null seriously. (in other words, try to avoid it at all costs. 😉)