Add compat option

[eemeli]

Fixes #329

Add a compatibility checker to the schema. The API is a bit different from that proposed in the issue:

export type SchemaOptions = {
  /**
   * When parsing, warn about compatibility issues with the given schema.
   * When stringifying, use scalar styles that are parsed correctly
   * by the `compat` schema as well as the actual schema.
   *
   * Default: `null`
   */
  compat?: string | Tags | null,
  ...
}

This is how it works:

import { parse } from 'yaml'

parse('x: true\ny: no', { compat: 'yaml-1.1' })
> { x: true, y: 'no' }

[TAG_RESOLVE_FAILED] YAMLWarning: Value may be parsed as either !!str or !!bool at line 2, column 1:

x: true
y: no
^

[TAG_RESOLVE_FAILED] YAMLWarning: Value may be parsed as either !!str or !!bool at line 2, column 4:

y: no
   ^^

This option serves at least these two different audiences:

• When parsing, it may be used e.g. in a linter or other validator to note values that may be parsed differently by different libraries.
• When stringifying, it may be used to output YAML that has a very high likelihood of being parsed as intended in libraries that use a different schema.