fix: always add index to entry (#2442)

59497d6 introduced `sorters.fuzzy_with_index_bias`, which gives a
scoring boost to earlier entries.

However, this sorter relies on an `index` key existing for the entry, which is
only populated by the static finder currently. We should set it from the
other finders, too.

This will allow us to use said sorter everywhere. It will also let us
replicate the behaviour of `fzf --tiebreak=index`:

```
  return pickers.new(opts, {
    finder = finders.new_oneshot_job(...)
    sorter = telescope.extensions.fzf.native_fzf_sorter(),
    tiebreak = function(current_entry, existing_entry, _)
      return current_entry.index < existing_entry.index
    end
  }):find()
```

This gives me better results for my "recently opened files" picker.
Other builtin pickers might benefit from this, too.

lua/telescope/finders.lua

@@ -65,16 +65,22 @@ function JobFinder:_find(prompt, process_result, process_complete)
     self.job:shutdown()
   end
 
+  local line_num = 0
   local on_output = function(_, line, _)
+    line_num = line_num + 1
     if not line or line == "" then
       return
     end
 
+    local entry
     if self.entry_maker then
-      line = self.entry_maker(line)
+      entry = self.entry_maker(line)
+      entry.index = line_num
+    else
+      entry = line
     end
 
-    process_result(line)
+    process_result(entry)
   end
 
   local opts = self:fn_command(prompt)
@@ -131,8 +137,12 @@ end
 function DynamicFinder:_find(prompt, process_result, process_complete)
   local results = self.fn(prompt)
 
+  local result_num = 0
   for _, result in ipairs(results) do
-    if process_result(self.entry_maker(result)) then
+    result_num = result_num + 1
+    local entry = self.entry_maker(result)
+    entry.index = result_num
+    if process_result(entry) then
       return
     end
   end
@@ -204,8 +214,13 @@ finders.new_table = function(t)
   return async_static_finder(t)
 end
 
-finders.new_dynamic = function(t)
-  return DynamicFinder:new(t)
+--- Used to create a finder from a function.
+--
+---@param opts table: stuff
+--         @key fn function() => list[string]
+--         @key entry_maker function Optional: function(line: string) => table
+finders.new_dynamic = function(opts)
+  return DynamicFinder:new(opts)
 end
 
 return finders