EagleFiler can display and search .ichat and .chat files created by Messages and iChat. Messages automatically archives iMessage conversations in dated folders at:
/Users/<username>/Library/Containers/com.apple.iChat/Data/Library/Messages/Archive/
To import these archives, just drag and drop the folders or individual files into EagleFiler.
Note: The Messages app does not save all attached files to the conversations archive. Attachments that are not archived there will not be imported into EagleFiler. Also, the macOS 11 version of Messages does not seem to save chat transcript files to disk.
You can also use the iMazing app to directly copy messages from an iOS device to your Mac and then import them into EagleFiler.