feat: Add nil-safety for fetching fields

[sonalys]

Summary

Allow fetching undefined/nil fields, by using expr.NilSafe().
Example:

package main

import (
	"encoding/json"
	"fmt"

	"github.com/expr-lang/expr"
)

func main() {
	got := any(nil)

	remapProgram := `
	let activeItems = filter(cart, .active == true);
	{
		testMap:     map(nonExistent.field, .attr),
		testFilter:  filter(nonExistent.field, .attr == "value"),
		testConcatUndefined: "Value: " + string(nonExistent.field),
		testGT: (nonExistent.field ?? 0) > 500,
	}
	`

	program, err := expr.Compile(remapProgram,
		expr.NilSafe(),
	)
	if err != nil {
		panic(err)
	}

	output, err := expr.Run(program, got)
	if err != nil {
		panic(err)
	}

	finalJSON, _ := json.MarshalIndent(output, "", "  ")
	fmt.Println(string(finalJSON))
}

// Output:
// {
//   "testConcatUndefined": "Value: \u003cnil\u003e",
//   "testFilter": [],
//   "testGT": false,
//   "testMap": []
// }

Details

• Modify runtime.Fetch to use sentinel instead of panic
• Ensure OpBegin works with type nil for reflect.Value.Len
• Improve performance with early return for nil value in builtin.lib.go.get
• Add expr.NilSafe() configuration

Open Questions

• Maybe expr.AllowUndefinedVariables() would be a better fit for the configuration
• I'm not sure how to properly test the implemented functionality, I kindly ask for feedback

Issues: #434