feat(sorters): allow user sort_by

* feat: Mixin Sorter (#1565) Self Solved

adding `mixin` sort options for `rust` like package systems

```

package.rs
package/
  __inside__

lib.rs
lib/
  _inside_

a.rs
b.rs
module.rs

```

* feat: sort_by, after_sort options for more convinient using

```
*nvim-tree.sort_by*
Changes how files within the same directory are sorted.
Can be one of 'name', 'case_sensitive', 'modification_time' or 'extension',
'function'.
>
  sort_by = function(a, b)
    if not (a and b) then
      return true
    end
    if a.nodes and not b.nodes then
      return true
    elseif not a.nodes and b.nodes then
      return false
    end

    return a.name:lower() <= b.name:lower()
  end

  end
  Type: `string | function(a, b)`, Default: `"name"`

*nvim-tree.after_sort*
Related to nvim-tree.sort_by, this function runs without mergesort.
Can be defined by your own after-sort works.
  Type: `function(table)`, Default: `disable`

>
  after_sort = function(t)
    local i = 1

    while i <= #t do
      if t[i] and t[i].nodes then
        local j = i + 1
        while j <= #t do
          if t[j] and not t[j].nodes and t[i].name:lower() == t[j].name:lower():match "(.+)%..+$" then
            local change_target = t[j]
            table.remove(t, j)
            table.insert(t, i, change_target)
            break
          end
          j = j + 1
        end
      end
      i = i + 1
    end
  end

```

* remove: after_sort ( misunderstood feature )

sort_by parameter can be function.

``` lua
  sort_by = function(t)
    local sorters = require "nvim-tree.explorer.sorters"
    local comparator = sorters.retrieve_comparator("name")
    sorters.split_merge(t, 1, #t, comparator) -- run default merge_sort
    local i = 1

    while i <= #t do
      if t[i] and t[i].nodes then
        local j = i + 1
        while j <= #t do
          if t[j] and not t[j].nodes and t[i].name:lower() == t[j].name:lower():match "(.+)%..+$" then
            local change_target = t[j]
            table.remove(t, j)
            table.insert(t, i, change_target)
            break
          end
          j = j + 1
        end
      end
      i = i + 1
    end
  end,

```

* try-fix: change existing merge_sort function, call user's sort_by

hope.. like it...?

* doc: explain function parameter and return, add more complex example

* fix: reorder with user-comparator exceed memory limit

apply merge_sort
check nil & type for senitize

* fix: user_index based sorting ( create index )

for performance, create index once,
using index to re-ordering

* fix: fence problems

* doc & fix: merge_sort problem fix & nil sorting

add complex example

* fix: sort_by detect and use string and nil

* doc: revert sort_by to simple

* fix: sort_by does not return anything

Co-authored-by: Alexander Courtis <alex@courtis.org>

lua/nvim-tree.lua

@@ -640,6 +640,7 @@ local FIELD_OVERRIDE_TYPECHECK = {
   height = { string = true, ["function"] = true, number = true },
   remove_keymaps = { boolean = true, table = true },
   on_attach = { ["function"] = true, string = true },
+  sort_by = { ["function"] = true, string = true },
 }
 
 local function validate_options(conf)