feat(code search): replace fuzzy search with union search for indexer (#6947)

Fuzzy searching for code has been known to be problematic #5264 and in my personal opinion isn't very useful.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/6947
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: Shiny Nematoda <snematoda.751k2@aleeas.com>
Co-committed-by: Shiny Nematoda <snematoda.751k2@aleeas.com>

modules/indexer/code/search.go

@@ -35,7 +35,14 @@ type SearchResultLanguages = internal.SearchResultLanguages
 
 type SearchOptions = internal.SearchOptions
 
-var CodeSearchOptions = [2]string{"exact", "fuzzy"}
+var CodeSearchOptions = [2]string{"exact", "union"}
+
+type SearchMode = internal.CodeSearchMode
+
+const (
+	SearchModeExact = internal.CodeSearchModeExact
+	SearchModeUnion = internal.CodeSearchModeUnion
+)
 
 func indices(content string, selectionStartIndex, selectionEndIndex int) (int, int) {
 	startIndex := selectionStartIndex
@@ -206,7 +213,6 @@ func searchResult(result *internal.SearchResult, startIndex, endIndex int) (*Res
 }
 
 // PerformSearch perform a search on a repository
-// if isFuzzy is true set the Damerau-Levenshtein distance from 0 to 2
 func PerformSearch(ctx context.Context, opts *SearchOptions) (int, []*Result, []*SearchResultLanguages, error) {
 	if opts == nil || len(opts.Keyword) == 0 {
 		return 0, nil, nil, nil