feat(java): enable .java filename completion for java command

[jiri-pejchal]

Modify java command completion scripts to also complete filenames with the .java extension. This adds support for

• JEP 330: Launch Single-File Source-Code Programs
• JEP 458: Launch Multi-File Source-Code Programs

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edited by @akinomyoga

Fixes #1196

[jiri-pejchal]

This pull request incorporates changes according to the commentary from #662 .

[akinomyoga]

The essential fix seems to be the same as https://github.com/scop/bash-completion/pull/662#issuecomment-1001646441. I prefer to leave the credit to the original author of #662, e.g., by including the Co-authored-by: ... in the commit message.

[jiri-pejchal]

• fixed tests when can_list_jar is False
• @frankslin added to Co-authored-by