refactor: Rename TypeScript type names to indicate nullability

[coderabbitai]

Context

In PR #42, we discussed renaming TypeScript type names to better indicate their nullability. The change involves:

1. Renaming FilteredLogEventMapTsType to include "Nullable" in its name for consistency with NullableLogEventIdx
2. Making similar changes in the yscope-log-viewer repository where these types are also defined

References

• Discussion: /feat: Add support for finding the log event that's closest to a target timestamp. #42#discussion_r1934294078

Tasks

• Rename FilteredLogEventMapTsType in clp-ffi-js
• Update corresponding type name in yscope-log-viewer
• Update any related documentation