Improve generated types for unknown schema objects

[drwpow]

Fixes #693, #554

Background

According to #693, I believe this is correct:

According to the OpenAPI spec, when a schema is defined without a type it is the equivalent of:

     anyOf:
       - type: string
       - type: number
       - type: integer
       - type: boolean
       - type: array
         items: {}
       - type: object

but openapi-typescript uses object type as the default and generates { [key: string]: unknown }. Instead it should generate just unknown.

This PR addresses that, fixing some bugs in the process. And also switches from any to unknown in other areas (#554).

I had originally thought that changing to unknown would be disruptive for people, but after looking again at the output I don’t believe it will be too bad for most. It should actually surface errors that people weren’t aware of before, which is the whole goal! So this will be released as a minor version rather than a breaking major version.

[codecov]

Codecov Report

Merging #769 (6aff470) into main (091b053) will increase coverage by 0.08%.
The diff coverage is 94.11%.

@@            Coverage Diff             @@
##             main     #769      +/-   ##
==========================================
+ Coverage   94.15%   94.24%   +0.08%     
==========================================
  Files          13       13              
  Lines         565      573       +8     
  Branches      200      204       +4     
==========================================
+ Hits          532      540       +8     
  Misses         32       32              
  Partials        1        1              

Impacted Files	Coverage Δ
src/utils.ts	95.45% <90.90%> (+0.45%)	⬆️
src/load.ts	82.78% <100.00%> (+0.28%)	⬆️
src/transform/schema.ts	99.00% <100.00%> (ø)

---

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 4ae2277...6aff470. Read the comment docs.

[drwpow]

This is a big improvement! unknown should be the correct type here.