[Repo Assist] perf/refactor: convert ILFieldDefs/ILEventDefs/ILPropertyDefs to concrete classes with lazy name-lookup caches

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Summary

This PR converts ILFieldDefs, ILEventDefs, and ILPropertyDefs from abstract interfaces to concrete classes, mirroring the existing ILMethodDefs pattern. It adds lazy O(1) name-lookup caches throughout and makes entries truly lazy (computed at most once per type).

Background

Previously these three types were abstract interfaces:

// Before — computed on every .Entries access; linear O(n) name lookups everywhere
type ILFieldDefs =
    abstract Entries: ILFieldDef[]

Every name-based lookup (GetField, GetPropertyImpl, GetEvent, GetEnumUnderlyingType) called Array.tryFind — an O(n) linear scan. Additionally, seekReadEvents and seekReadProperties recomputed the entries array on every .Entries access (the binary reader range scan ran again each time).

Changes

Concrete classes with lazy dict caches — mirroring ILMethodDefs:

type ILFieldDefs(larr: Lazy<ILFieldDef[]>) =
    let lmap = lazy (
        let d = Dictionary<string, ILFieldDef>()
        for f in larr.Force() do d.[f.Name] <- f
        d)
    member __.Entries = larr.Force()
    member __.TryFindByName nm =
        let scc, v = lmap.Value.TryGetValue(nm) in if scc then Some v else None

Similarly for ILEventDefs and ILPropertyDefs.

Entries are now truly lazy — seekReadEvents/seekReadProperties previously re-ran the range scan on every .Entries call. Now the lazy constructor caches after first access.

O(1) lookups in TypeSymbol and TargetTypeDefinition:

• TypeSymbol.GetField/GetPropertyImpl/GetEvent (generic instantiations): now use TryFindByName instead of Array.tryFind
• TargetTypeDefinition.GetField/GetPropertyImpl/GetEvent: new lazy fieldDefsMap/propDefsMap/eventDefsMap dictionaries over the wrapped reflection objects
• TargetTypeDefinition.GetEnumUnderlyingType: uses TryFindByName for the "value__" field

All creation sites updated: empty constants, seekReadFields/seekReadEvents/seekReadProperties, and ProvidedTypeBuilder.

Trade-offs

• Slightly higher memory use per loaded ILTypeDef (dictionary overhead), but only allocated on first name lookup
• No API surface changes — Entries still works identically; TryFindByName is a new additive method

Relation to issue #500

This PR implements the code improvements from the blocked issue #500 (which was blocked by the inclusion of .github/dependabot.yml). This PR contains only the code changes, without the Dependabot config.

Test Status

✅ 126/126 tests pass (dotnet test tests/FSharp.TypeProviders.SDK.Tests.fsproj -c Release --framework net8.0)

Build: dotnet build src/FSharp.TypeProviders.SDK.fsproj -c Release — succeeded, 0 warnings, 0 errors.

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