rfc: add DeepAssign

[cdaringe]

Is your feature request related to a real problem or use-case?

When I have multiple sources of options that need to be combined,
I want to use a deepmerge,
but the resulting type is imperfect. That is, there is not currently available a generic type that properly represents a deep merging of objects with non-uniform shape.

This is a common use case exemplified by the 20million d/ls on deepmerge

Example shown in the solution below.

Describe a solution including usage in code example

functional example

Who does this impact? Who is this for?

TS users wanting to merge deep objects and get a correct type out.

Describe alternatives you've considered (optional)

• creating explicit types and performing the nested merges manually
• just flattening stuff and avoiding deep merging altogether
• using a DeepPartial, then using something like zod or runtypes to reflect back the deep type at the cost of runtime perf

Additional context (optional)

n/a

[piotrwitek]

Looks useful to me, please feel free to open PR. Thanks