feat(syncthing): add 3rd party fallback compmletion loader

[scop]

https://syncthing.net (no online documentation for installing completions that I can find)

[akinomyoga]

I think it is good to include the URL to the top page in the commit message even if it's not a link to the documentation of the completion setup.

Or one may include the link to the corresponding line in the source code:

https://github.com/syncthing/syncthing/blob/1bba07a15210fcbdb21b98f12211cd9d8dc07ca5/cmd/syncthing/main.go#L142

By the way, I noticed a typo in the commit message: comp{m => }letion.

Otherwise, approving.

[akinomyoga]

The completion support seems to be based on Kongplete. Maybe we can mention this in the commit message and also in the completion file.

However, the feature to print the completion setting by install-completions seems to be provided by the unreleased Kongplete-0.3.0. The Kongplete documentation doesn't mention the usage of install-completions at all. According to WillAbides/kongplete#14, the feature to print the completion setting (so that one can eval or source the output) is only in 0.3.0 and missing in the latest official release Kongplete-0.2.0.

• Syncthing seems to rely on Kongplete 0.4.0.
• Another project Terramate also seems to offer the Kongplete-based completion support. However, the usage of terramate install-completions described in its documentation seems different from the one in this PR. A similar description is also found in this page in the documentation. The completion support was added in terramate-io/terramate#96. I realized that Terramate seems to depend on Kongplete 0.2.0. The usage difference seems to come from the difference in the Kongplete version.
• Even another project oalders/is¹ also uses the Kongplete-based completion. The usage described in its documentation is actually consistent with this PR. This depends on Kongplete 0.3.0.

Footnotes

1. this project oalders/is has only 47 stars but has a conflicting command name with microsoft/inshellisense, so I don't think we should cover oalders/is for the third-party completion loader. I've mentioned this project just for the usage of install-completions described in its documentation. ↩