Various fixes/improvements.

[PJB3005]

Sorry for this not being separate PRs, I initially wanted to fix more about the library and figured all the fixes would interconflict anyways so this would be saner. Then I realized the problem is that this library is built on an ancient pre-1.0 version of fluent.js that itself was a complete trash fire code wise so I kind of gave up midway fixing more.

Frankly this library would be better served being re-ported from the current TypeScript or Rust implementations since dear god this old JS implementation is awful but I don't really have the motivation to do such an undertaking, so...

This PR still contains some useful fixes/improvements though that I could use so I figured I'd still submit it.

The individual commits have the individual changes, so look at those.

Should be no backwards compatibility breakages (there is an [Obsolete] now though).

[codecov[bot]]

Codecov Report

Merging #9 (b790b86) into master (3fb1005) will increase coverage by 0.09%. The diff coverage is 93.23%.

@@            Coverage Diff             @@
##           master       #9      +/-   ##
==========================================
+ Coverage   93.61%   93.70%   +0.09%     
==========================================
  Files          26       30       +4     
  Lines        1941     1890      -51     
==========================================
- Hits         1817     1771      -46     
+ Misses        124      119       -5     

Impacted Files	Coverage Δ
Fluent.Net/BuiltIns.cs	60.00% <60.00%> (ø)
Fluent.Net/Resolver.cs	88.96% <84.21%> (+1.37%)	:arrow_up:
Fluent.Net/Plural/LocaleRules.cs	90.90% <87.50%> (-5.76%)	:arrow_down:
Fluent.Net/MessageContext.cs	90.69% <92.30%> (-3.55%)	:arrow_down:
Fluent.Net/FluentError.cs	100.00% <100.00%> (ø)
Fluent.Net/FluentResource.cs	100.00% <100.00%> (ø)
Fluent.Net/Types.cs	100.00% <100.00%> (ø)
Fluent.Net/ParserStream.cs	85.56% <0.00%> (-0.71%)	:arrow_down:
Fluent.Net/Serializer.cs	91.51% <0.00%> (-0.15%)	:arrow_down:
Fluent.Net/FtlParserStream.cs	91.80% <0.00%> (-0.09%)	:arrow_down:
... and 9 more

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