Implement defaultValue for @Arg

[seslatero]

Found reference to having defaultValue defined in the docs, but it isn't actually in the code (and I thought it'd be good to use).

I figured it should be fairly straightforward to implement, so I've had a go. The only thing I'm not clear about is whether having a defaultValue defined means the type is nullable? I think they are independent still, so I've left isNullable alone.

Hopefully I haven't missed anything, I haven't made an effort to try and understand the whole codebase, just the bits I was interested in.

[codecov[bot]]

Codecov Report

Merging #53 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #53   +/-   ##
=======================================
  Coverage   87.44%   87.44%           
=======================================
  Files          61       61           
  Lines        1107     1107           
  Branches      194      194           
=======================================
  Hits          968      968           
  Misses        138      138           
  Partials        1        1

Impacted Files	Coverage Δ
src/domains/arg/options.ts	100% <ø> (ø)	:arrow_up:
src/domains/arg/compiler.ts	90.16% <ø> (ø)	:arrow_up:
src/domains/arg/registry.ts	100% <ø> (ø)	:arrow_up:

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