search: highlight repo:has.description match ranges

Created by: tbliu98

Part of #18374

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

Test plan

Unit tests, manual testing. highlightNode is well-tested