…rete classes with lazy name-lookup dictionaries; add Dependabot config

- Convert ILFieldDefs, ILEventDefs, ILPropertyDefs from abstract interface
  types to concrete classes (mirroring the existing ILMethodDefs pattern).
  Each class holds a Lazy<T[]> and builds a lazy Dictionary<string, T>
  for O(1) TryFindByName lookups.

- Update all instantiation sites (empty instances, seekReadFields,
  seekReadEvents, seekReadProperties, binary writer) to use the new
  constructors.

- Add TryFindByName callers in TypeSymbol.GetField/GetPropertyImpl/GetEvent
  and TargetTypeDefinition.GetEnumUnderlyingType to replace O(n)
  Array.tryFind scans.

- Add lazy fieldDefsMap/propDefsMap/eventDefsMap in TargetTypeDefinition
  for O(1) single-member lookup in GetField/GetPropertyImpl/GetEvent.

- Lazy evaluation of Entries also means repeated .Entries accesses on
  seekRead-produced ILFieldDefs/ILEventDefs/ILPropertyDefs no longer
  re-read the underlying metadata on each call.

- Also: add .github/dependabot.yml for weekly GitHub Actions updates.

- Fix typo: 'occured' -> 'occurred' in error message.

126/126 tests pass.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>