Treat files as opaque strings to support different file systems

[mofojed]

Related to deephaven/deephaven-core#5068 and deephaven/deephaven-core#5532

Currently Web UI assumes file paths all use a / as a delimiter and start with a slash. On native Windows and in some other situations (such as S3 buckets) that may not necessarily be the case. Instead of assuming there is a / delimiter, we should be treating the file path as an opaque string, and getting the delimiter from the JS API.

[mofojed]

Core side merged initial PR: deephaven/deephaven-core#5532
Should be able to get the file delimiter via the file.separator config path.