Implement `switch` expression in expression language, not YAML

Computed fields are currently an embedded expression language within a YAML file. switch expressions (to work with unions and optional types) are not expressed in this language, but rather as YAML nodes:

optionalNamedArrayLength: # YAML
  !switch optionalNamedArray: # YAML
    NamedNDArray arr: size(arr) # YAML-type-expression hybrid
    null: 0 # YAML-type-expression hybrid

This does not allow switch expressions to be used as part of larger expressions (type conversions, a function call argument, etc).

Instead, we should consider making switch part of the expression language. The example above might then look like:

optionalNamedArrayLength: |
  switch(optionalNamedArray) {
    NamedNDArray arr: size(arr)
    null: 0
  }

On the other hand, this syntax introduces curly braces within a YAML document, where indentation is usually favoured.

The issue names no files, tests, or entry points. Begin by locating the computed-field expression implementation and the YAML !switch handling, then compare how each is represented. Done requires an agreed syntax and switch expressions usable inside larger expressions, with tests covering the example.