revert completion scripts removal (#808)

partial revert of: https://gitea.com/gitea/tea/pulls/782
closes: https://gitea.com/gitea/tea/issues/784

Old versions of tea have hardcoded completion fetched from main branch

Those should not be used from v0.10 onward.

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Reviewed-on: https://gitea.com/gitea/tea/pulls/808
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: TheFox0x7 <thefox0x7@gmail.com>
Co-committed-by: TheFox0x7 <thefox0x7@gmail.com>

contrib/autocomplete.ps1

@@ -0,0 +1,9 @@
+$fn = $($MyInvocation.MyCommand.Name)
+$name = $fn -replace "(.*)\.ps1$", '$1'
+Register-ArgumentCompleter -Native -CommandName $name -ScriptBlock {
+     param($commandName, $wordToComplete, $cursorPosition)
+     $other = "$wordToComplete --generate-bash-completion"
+         Invoke-Expression $other | ForEach-Object {
+            [System.Management.Automation.CompletionResult]::new($_, $_, 'ParameterValue', $_)
+         }
+ }