Unit tests for PropExpressions to go to/from string representation + single StructuredDataPropExpression::from(…) method

Problem/Motivation

It's become clear over the past 24 hours that FieldPropExpression and friends (everything in src/PropExpressions) is hard to grok for people.

Eventually we'll need docs, initially unit tests can serve as docs.

In this first pass, there is one key goal: unit tests to go from PHP object representation to string representation and vice versa should pass — because once that's working, contributors won't lose time anymore due to bugs in that low-level foundation. (Which currently still is 99% identical to the PoC from ~1.5 month ago.)

Out of scope: making all code in src/PropExpressions beautiful/elegant/DRY/… — the purpose is only the bold bit above, because that A) unblocks other issues/MRs, B) keeps scope tight.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

• API addition: single StructuredDataPropExpression::from(…) method to avoid if/else logic to invoke the appropriate class-specific method.

Data model changes

• 🥳 No more mysterious invisible whitespace in the string representation. Result: less frustration, and the sort order in test expectations now actually makes sense.
• The string representation of Field(Type)ObjectPropsExpression no longer lists the mapped object props separated by , (comma + space), but only by , (comma).