refactor: remove file token feature

Problem: The file metadata token (file:<path>:<line>) was implemented
but is no longer wanted.

Solution: Remove all traces — parse.lua token parsing, diff.lua
reconciliation, views.lua LineMeta field, buffer.lua virtual text and
PendingFile highlight, complete.lua omnifunc trigger, init.lua
goto_file/add_here functions and -file edit token, plugin keymaps
<Plug>(pending-goto-file) and <Plug>(pending-add-here), config.lua
goto_file keymap field, vimdoc FILE TOKEN section, and
spec/file_spec.lua.

lua/pending/config.lua

@@ -31,7 +31,6 @@
 ---@field prev_header? string|false
 ---@field next_task? string|false
 ---@field prev_task? string|false
----@field goto_file? string|false
 
 ---@class pending.Config
 ---@field data_path string