feat(core): Add SPC f a key binding to save as new file

Problem: There is no key binding to save current buffer as new file.
Solution: Add `SPC f a` key binding

close https://github.com/SpaceVim/SpaceVim/issues/4310

autoload/SpaceVim/layers/core.vim

@@ -149,6 +149,9 @@ function! SpaceVim#layers#core#config() abort
   call SpaceVim#mapping#space#def('nnoremap', ['f', 's'], 'call call('
         \ . string(s:_function('s:save_current_file')) . ', [])',
         \ 'save-current-file', 1)
+  call SpaceVim#mapping#space#def('nnoremap', ['f', 'a'], 'call call('
+        \ . string(s:_function('s:save_as_new_file')) . ', [])',
+        \ 'save-as-new-file', 1)
   call SpaceVim#mapping#space#def('nnoremap', ['f', 'S'], 'wall', 'save-all-files', 1)
   " help mappings
   call SpaceVim#mapping#space#def('nnoremap', ['h', 'I'], 'call SpaceVim#issue#report()', 'report-issue-or-bug', 1)
@@ -936,6 +939,34 @@ function! s:save_current_file() abort
   endif
 endfunction
 
+function! s:save_as_new_file() abort
+  let current_fname = bufname()
+  if !empty(current_fname)
+    let dir = fnamemodify(current_fname, ':h') . s:FILE.separator
+  else
+    let dir = getcwd() . s:FILE.separator
+  endif
+  let input = input('save as: ', dir, 'file')
+  " clear cmdline
+  noautocmd normal! :
+  if !empty(input)
+    exe 'silent! write ' . input
+    exe 'e ' . input
+    if v:errmsg !=# ''
+      echohl ErrorMsg
+      echo  v:errmsg
+      echohl None
+    else
+      echohl Delimiter
+      echo  fnamemodify(bufname(), ':.:gs?[\\/]?/?') . ' written'
+      echohl None
+    endif
+  else
+    echo 'canceled!'
+  endif
+
+endfunction
+
 let g:_spacevim_autoclose_filetree = 1
 function! s:explore_current_dir(cur) abort
   if g:spacevim_filemanager ==# 'vimfiler'

docs/cn/documentation.md

@@ -1334,6 +1334,7 @@ SpaceVim 选项 `window_leader` 的值来设为其它按键：
 | `SPC f o`            | 代开文件树，并定位到当前文件                           |
 | `SPC f R`            | rename the current file(TODO)                          |
 | `SPC f s` / `Ctrl-s` | 保存文件 (:w)                                          |
+| `SPC f a`            | 另存为新的文件                                         |
 | `SPC f W`            | 使用管理员模式保存                                     |
 | `SPC f S`            | 保存所有文件                                           |
 | `SPC f r`            | 打开文件历史                                           |

docs/documentation.md

@@ -1424,6 +1424,7 @@ Files manipulation commands (start with `f`):
 | `SPC f o`    | Find current file in file tree                                          |
 | `SPC f R`    | rename the current file(TODO)                                           |
 | `SPC f s`    | save a file                                                             |
+| `SPC f a`    | save as new file name                                                   |
 | `SPC f S`    | save all files                                                          |
 | `SPC f r`    | open a recent file                                                      |
 | `SPC f t`    | toggle file tree side bar                                               |