Make *AtPos functions work for incomplete config

[radeksimko]

HCL Template

provider ""

Parsing logic

package main

import (
	"log"
	
	"github.com/kr/pretty"
	"github.com/hashicorp/hcl/v2"
	"github.com/hashicorp/hcl/v2/hclsyntax"
)

func main() {
	c, diags := hclsyntax.ParseConfig([]byte(`
provider ""
`), "test.tf", hcl.InitialPos)
	if diags.HasErrors() {
		log.Printf("diags: %s", diags.Error())
	}

	log.Printf("config: %s", pretty.Sprint(c))

	b := c.OutermostBlockAtPos(hcl.Pos{
		Line:   2,
		Column: 11,
		Byte:   13,
	})

	log.Printf("block: %s", pretty.Sprint(b))
}

Expected behavior

OutermostBlockAtPos in the above example returns the incomplete block.

Actual behavior

OutermostBlockAtPos in the above example returns nil.

Steps to reproduce

go run the above code snippet

Use Cases

When users are editing config and ask the language server via the IDE for completion, it's expectable they will want to complete an incomplete block, such as the above and the language server will therefore need to know where exactly in the incomplete block they're requesting completion.