Add better type for flags with default value

[frangio]

A flag with a default value was being typed as as T | undefined. For example the following would not type:

      const out = parse([], {
        flags: {
          foo: flags.string({ default: 'baz' }),
          bar: flags.integer({ default: 42 }),
        }
      })
      const foo: string = out.flags.foo;
      const bar: number = out.flags.bar;

I added a special case in the type for this, the same way that required: true works.

Making the flag required is a workaround to get the above to type, but this is undesirable because it incorrectly shows the flag as required in the help.

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @frangio to sign the Salesforce.com Contributor License Agreement.

[codecov]

Codecov Report

Merging #53 into master will decrease coverage by 91.35%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #53       +/-   ##
========================================
- Coverage   91.35%    0%   -91.36%     
========================================
  Files          11     1       -10     
  Lines         324    29      -295     
  Branches       85    14       -71     
========================================
- Hits          296     0      -296     
- Misses         12    29       +17     
+ Partials       16     0       -16

Impacted Files	Coverage Δ
src/validate.ts	0% <0%> (-96.88%)	⬇️
src/deps.ts
src/index.ts
src/help.ts
src/util.ts
src/list.ts
src/screen.ts
src/args.ts
src/errors.ts
src/parse.ts

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f3880ae...9add94f. Read the comment docs.

[frangio]

Not sure why the CI is not running. A previous run had passed.

[frangio]

@RasPhilCo any chance to get this small PR merged? 🙏

Thanks!

[RasPhilCo]

Nice! Looking into why Circle stopped running... Circle is back.

[frangio]

The CI failure seems unrelated to this PR. master is failing for the same reason. What do you suggest? I can look into fixing it. The problem is an implicit any type.

[RasPhilCo]

Can you push another commit with a comment or something small to trigger CI again? I had to re-add the repo and I think there's a caching issue.

[frangio]

I fixed what was causing the tests to fail.

This coverage report is very odd-looking though. 🤔 -91.36%?

[RasPhilCo]

🏄