Add ToLower, Trim, ToDecimal transforms and AddJsonTransform DI convenience method

[Copilot]

JSLTSharp lacked several common string/numeric transforms and required two separate DI registration calls to wire up the engine.

New built-in transformation operations

• ToLower — lower-cases a string field (complement to ToUpper)
• Trim — strips leading/trailing whitespace from a string field
• ToDecimal — converts string, integer, float, or boolean to decimal; returns null on unconvertible input (complement to ToInteger)

All three are registered automatically via the existing RegisterJsonCustomTransformFunctions().

Simplified DI registration

New AddJsonTransform() extension method collapses the previous two-step setup into one:

// Before
services.RegisterJsonCustomTransformFunctions();
services.AddSingleton<JsonTransform>();

// After
services.AddJsonTransform();

Usage examples

{ "normalized": "$.rawField->Trim()->ToLower()" }
{ "price":      "$.rawPrice->ToDecimal()" }