local or remote compress

[LiNk-NY]

• *support local and remote compressed json files
• re-use checks for url and file
• update documentation
• import file_ext() from tools

[mtmorgan]

The problem with the PR is that it believes the file name to accurately describe the file content. The pkgdown failure at https://github.com/mtmorgan/rjsoncons/actions/runs/19049336654/job/54405421923?pr=5#step:7:91 occurs because the file says it is '.json.gz' but is actually an ndjson. Apparently this is / was a common practice. So I would rather the PR actually believes the file content rather than the file name.

I guess this has necessary performance consequences for compressed files, especially when remote, because it requires that the file be at least partly decompressed, and hence downloaded, to determine its type. But maybe this better practice anyway -- a local cache of the file while being processed?