cmd/syncthing: Add CLI completion functionality (fixes #8616) (#9226)

### Purpose

This implements CLI completion using the Kongplete module. As a side
effect a CLI structure for syncthing/cli was created for kongplete to be
able to parse and implement CLI completion.

### Testing

I've tested the autocompletion manually, and it had worked, but I hadn't
added any tests so as to test it automatically. Additionally, I ran `go
run build.go test` with all tests passing.

go.mod

@@ -37,6 +37,7 @@ require (
 	github.com/thejerf/suture/v4 v4.0.2
 	github.com/urfave/cli v1.22.14
 	github.com/vitrun/qart v0.0.0-20160531060029-bf64b92db6b0
+	github.com/willabides/kongplete v0.3.0
 	golang.org/x/crypto v0.17.0
 	golang.org/x/exp v0.0.0-20240103183307-be819d1f06fc
 	golang.org/x/net v0.19.0
@@ -64,6 +65,8 @@ require (
 	github.com/golang/snappy v0.0.4 // indirect
 	github.com/google/pprof v0.0.0-20231229205709-960ae82b1e42 // indirect
 	github.com/google/uuid v1.5.0 // indirect
+	github.com/hashicorp/errwrap v1.0.0 // indirect
+	github.com/hashicorp/go-multierror v1.0.0 // indirect
 	github.com/klauspost/cpuid/v2 v2.2.6 // indirect
 	github.com/matttproud/golang_protobuf_extensions/v2 v2.0.0 // indirect
 	github.com/nxadm/tail v1.4.11 // indirect
@@ -71,11 +74,13 @@ require (
 	github.com/onsi/gomega v1.30.0 // indirect
 	github.com/oschwald/maxminddb-golang v1.12.0 // indirect
 	github.com/pkg/errors v0.9.1 // indirect
+	github.com/posener/complete v1.2.3 // indirect
 	github.com/power-devops/perfstat v0.0.0-20221212215047-62379fc7944b // indirect
 	github.com/prometheus/client_model v0.5.0 // indirect
 	github.com/prometheus/common v0.45.0 // indirect
 	github.com/prometheus/procfs v0.12.0 // indirect
 	github.com/quic-go/qtls-go1-20 v0.4.1 // indirect
+	github.com/riywo/loginshell v0.0.0-20200815045211-7d26008be1ab // indirect
 	github.com/russross/blackfriday/v2 v2.1.0 // indirect
 	github.com/yusufpapurcu/wmi v1.2.3 // indirect
 	go.uber.org/mock v0.4.0 // indirect