This PR adds highlighting for ranges in the visible portion of repo descriptions that are matched by repo:has.description.
During RepoSearchJob construction, compile any repo:has.description patterns into regexes that match case-insensitively and across newline characters
During RepoSearchJob execution, get resolved repositories' descriptions from the database, match them against the list of compiled regexes, and return the matched ranges as the field DescriptionMatches on result.RepoMatch
Add field DescriptionMatches to EventRepoMatch and populate it with the values from result.RepoMatch in frontend
Add field descriptionMatches to RepositoryMatch interface in the client
Use highlightNode to highlight matched ranges in the RepoSearchResult component
