Add TypeScript typings

[eemeli]

Fixes #102
Fixes DefinitelyTyped/DefinitelyTyped#33780
Closes DefinitelyTyped/DefinitelyTyped#43450
Related: #23, #47

These types are based on those published as @types/yaml, which are mostly the work of @ikatyang, with contributions from @ColinBradley and @shirk3y. In fact, I've picked out the original commits from DefinitelyTyped into this branch, and worked from there.

I'm relatively confident about these types, though I've not tested them very deeply. There are a few changes that may be relevant:

• The DT types advertise themselves as being valid for TS 2.2; these require at the very least TS 2.3 due to Iterable, possibly a bit later than that.
• Some of the interface and namespaces are available from different names and endpoints compared to before:

  // previously, with @types/yaml
  import { ast, cst, ParseOptions, YAMLError } from 'yaml'
  
  // now
  import { Options } from 'yaml'
  import { AST, YAMLError } from 'yaml/types'
  import { CST } from 'yaml/parse-cst'

• The TS class inheritance doesn't exactly match the JS implementation, but it's really close.
• I've not included typings for deprecated endpoints

The tsconfig.json and yaml-tests.ts files are remnants of the DT history, and should probably get removed before merging this.

Comments are very welcome here, in particular wrt changes to the DT types. For example, if there's reason to do so, re-exports could be added to match the previous typings.

[ikatyang]

I tested it in prettier/yaml-unist-parser@d999748, it looks good but it seems you forgot to add files: ["*.d.ts"] in package.json.

Some of the interface and namespaces are available from different names and endpoints compared to before

I'd recommend to export all the used types. For example, index.d.ts exports parseCST from parse-cst.d.ts so all the types that are related to parseCST would be better to be re-exported to index.d.ts as well.

[shirk3y]

Should we define interface for these options?

[eemeli]

This needs to be extendable by custom tags, each of which will have their own options shape. I'm not sure what specifying this further would really help?

[eemeli]

@ikatyang I added re-exports of the AST and CST namespaces to index.d.ts. All of the other types are tied pretty closely to the actual exports of yaml/types and yaml/util, so I don't think they really make sense being re-exported. Also good point about actually include the declarations in the package.