js-yaml breaking changes

Astral characters are no longer encoded by `dump()`, #587.

"duplicate mapping key" exception now points at the correct column, #452.

Extra commas in flow collections (e.g. `[foo,,bar]`) now throw an exception

Check [migration guide](migrate_v3_to_v4.md) to see details for all breaking changes.

`!!binary` now always mapped to `Uint8Array` on load.

Reduced nesting of `/lib` folder.

Parse numbers according to YAML 1.2 instead of YAML 1.1 (`01234` is now decimal,

`dump()` no longer quotes `:`, `[`, `]`, `(`, `)` except when necessary, #470, #557.

Line and column in exceptions are now formatted as `(X:Y)` instead of

Code snippet created in exceptions now contains multiple lines with line numbers.

`dump()` with `skipInvalid=true` now serializes invalid items in collections as null.

Custom tags starting with `!` are now dumped as `!tag` instead of `!<!tag>`, #576.

Custom tags starting with `tag:yaml.org,2002:` are now shorthanded using `!!`, #258.

Added `.mjs` (es modules) support.

Added `quotingType` and `forceQuotes` options for dumper to configure

Added `styles: { '!!null': 'empty' }` option for dumper

Added `replacer` option (similar to option in JSON.stringify), #339.

Custom `Tag` can now handle all tags or multiple tags with the same prefix, #385.

Breaking: removed `safe*` functions. Use `load`, `loadAll`, `dump`

`yaml.DEFAULT_SAFE_SCHEMA` and `yaml.DEFAULT_FULL_SCHEMA` are removed, use

`yaml.Schema.create(schema, tags)` is removed, use `schema.extend(tags)` instead.

`dump()` now serializes `undefined` as `null` in collections and removes keys with