This commit extends the type expression package to add two new features:

- In constraint mode, the `optional(...)` modifier can be used on object
  attributes to allow them to be omitted from input values to a type
  conversion process. Any such missing attributes will be replaced with
  a `null` value of the appropriate type upon conversion.
- In the new defaults mode, the `optional(...)` modifier takes a second
  argument, which accepts a default value of an appropriate type. These
  defaults are returned alongside the type constraint, and may be
  applied prior to type conversion through the new `Defaults.Apply()`
  method.

This change is upstreamed from Terraform, where optional object
attributes have been available for some time. The defaults functionality
is new and due to be released with Terraform 1.3.

typeexpr: Optional object attributes with defaults

When parsing optional object attribute defaults, we previously verified
that the default value was convertible to the attribute type. However,
we did not keep this converted value.

This commit uses the converted default value, rather than delaying
conversion until later. In turn this prevents crashes when transforming
collections which contain objects with optional attributes, caused by
incompatible object types at the time of defaults application.

ext/typeexpr: Convert defaults for optional attrs