Improve TS developer experience

[eemeli]

This packages in a small bunch of fixes discovered while porting the remaining JS tests to TS:

• Add Strict type flag to Composer, Document, parseDocument, and parseAllDocument

  The current typing of Document members and methods is pretty strict, as they account for e.g. doc.contents being possibly null, and for the get() and getIn() methods returning unknown. While technically accurate, these types are often a hindrance. To address this, let's add a second optional generic type argument Strict that relaxes these and a few other types:

  import { parseDocument, YAMLMap } from 'yaml'
  
  const strictDoc = parseDocument<YAMLMap>('foo: bar')
  strictDoc.contents // YAMLMap | null
  strictDoc.get('foo') // unknown
  
  const looseDoc = parseDocument<YAMLMap, false>('foo: bar')
  looseDoc.contents // YAMLMap
  looseDoc.get('foo') // any

• Add undefined & Date handling to the NodeType<T> utility type

• Export FoldOptions type from 'yaml/util'

• Convert all remaining JS tests to TS & include their type checking in the test:types script

• Use package.json "typesVersions" to find d.ts files from dist/

Making the typing of doc.directives (which could technically be null, but practically never is) depend on Strict ended up hitting an issue with TS 4.2 and older, which provided this useful and very sensible error:

Type 'boolean' is not assignable to type 'T extends true ? boolean | undefined : boolean'.

As I could not find any better workaround for this, the Document.d.ts file needs a /** @ts-ignore */ comment to deal with it, but that's unfortunately not parsed right in TS 3.8 and older (only JSDoc comments get added to d.ts files). It might be possible to fix this there via "typesVersions", but as far as I can tell that would require duplicating all of the 75 d.ts files just to be able to apply a single-line patch to the Document.d.ts as the version-dependent paths are only checked for entry points, and we're providing nearly everything from 'yaml'.

So instead this PR moves the minimum support level to TS 3.9 and adds a note to the readme and docs intro about this state of affairs:

The minimum supported TypeScript version of the included typings is 3.9; for use in earlier versions you may need to set skipLibCheck: true in your config. This requirement may be updated between minor versions of the library.