Make sure limited label candidates are sorted

[radeksimko]

This was caught by a flaky test on main after merging #85

=== RUN   TestDecoder_CandidateAtPos_incompleteLabels
    label_candidates_test.go:102: unexpected candidates:   lang.Candidates{
          	List: []lang.Candidate{
          		{
        - 			Label:        "first",
        + 			Label:        "second",
          			Description:  {},
          			Detail:       "",
          			IsDeprecated: false,
          			TextEdit: lang.TextEdit{
          				Range:   {Filename: "test.tf", Start: {Line: 1, Column: 14, Byte: 13}, End: {Line: 1, Column: 14, Byte: 13}},
        - 				NewText: "first",
        + 				NewText: "second",
        - 				Snippet: "first",
        + 				Snippet: "second",
          			},
          			AdditionalTextEdits: nil,
          			Kind:                s"LabelCandidateKind",
          			TriggerSuggest:      false,
          		},
          	},
          	IsComplete: false,
          }
--- FAIL: TestDecoder_CandidateAtPos_incompleteLabels (0.00s)

Prior to this patch we were ranging over a map which doesn't have a guaranteed order. The solution reflects what we already do for attributes and blocks - i.e. iterate over sorted keys instead of a map:

hcl-lang/decoder/body_candidates.go

Lines 20 to 21 in 5f3f065

	attrNames := sortedAttributeNames(schema.Attributes)
	for _, name := range attrNames {

hcl-lang/decoder/body_candidates.go

Lines 46 to 47 in 5f3f065

	blockTypes := sortedBlockTypes(schema.Blocks)
	for _, bType := range blockTypes {