Extract bean property path support into PropertyPath

[bclozel]

Currently bean property path support (for data binding) is currently implemented in several places like AbstractNestablePropertyAccessor and PropertyAccessorUtils. This includes turning the input string into a canonical representation of the property path (for caching and allowed/disallowed checks) and splitting the path into segments for target property access.

This enhancement should focus the implementation in a single place and avoid behavior differences between different parts of the codebase.

This is already work in progress so contributions are not welcome.

[bclozel]

The draft grammar looks like:

PropertyPath := Segment ('.' Segment)*
Segment      := Name Index*                  -- Name may be empty only if at least one Index follows
Name         := char* excluding '.', '[', ']'
Index        := '[' Key ']'
Key          := QuotedKey | RawKey
QuotedKey    := "'" [^']* "'"  |  '"' [^"]* '"'
RawKey       := char* excluding quote characters, with balanced '[' / ']' nesting

I think this reflects the main intent behind bean property paths and allows the vast majority of existing use cases.

Notable behavior changes:

Case	New behavior	Examples
Invalid property paths	Currently ignored or rejected at binding time, the new behavior rejects them at parsing time.	map[key1]other nestedMap[a]X[b] nested.].name
Single quote keys	Currently single quote keys are accepted, the new implementation requires them to be quoted. See gh-36765	map['] should be map["'"] map["] should be map['"']
Unbalanced quoted keys	Currently somewhat accepted, they are now rejected. See previous row.	map[don't] map[a'b] map['key1] map["key1]
Canonical form idempotent?	Currently it's not because of how quotes are managed in keys. New implementation guarantees it.	Previously: map[''a''] yields map['a'] and map['a'] yields map[a] Now map[''a''] is rejected.
Index characters in keys	Currently rejected, but with stricter quote support in keys this is now possible.	"map['a]b'] was rejected, it now binds