This PR add basic support to indents.

Some cases that have not been implemented:

Variant

type user = 
  | Id
  | Name

How to set the node/rule where indentation ends?

Labeled Variant Payloads (Inline Record)

| is the cursor

type user =
  | Id
  | Name({|})

Press enter:

type user =
  | Id
  | Name({
  |
  })

Here the indentation not work because Name({}) is a ERROR. record_type rule require a field

ERROR [0, 0] - [2, 12]
  type_identifier [0, 5] - [0, 9]
  variant_declaration [1, 4] - [1, 6]
    variant_identifier [1, 4] - [1, 6]
  variant_identifier [2, 4] - [2, 8]

Could record_type be an optional rule to not generate an ERROR node in the parser?