fix errors when processing files with backslashes in the path

[luozhiya]

Problem

When opening a file by dragging in Windows, fittencode.vim will report an error.

The log is as follows

Messages maintainer: The Vim Project
"C:\Hello" 0L, 0B
Error detected while processing function CodeCompletion:
line   58:
E716: Key not present in Dictionary: "generated_text"
line   59:
E121: Undefined variable: l:generated_text
E116: Invalid arguments for function substitute(l:generated_text, '<.endoftext.>', '', 'g')
line   61:
E121: Undefined variable: l:generated_text
E116: Invalid arguments for function empty(l:generated_text)
line   67:
E121: Undefined variable: l:generated_text
E116: Invalid arguments for function split(l:generated_text, "\n", 1)
line   68:
E121: Undefined variable: l:text
E116: Invalid arguments for function empty(l:text[-1])
line   73:
E121: Undefined variable: text
line   85:
E121: Undefined variable: l:generated_text

Steps to reproduce

• Install latest gvim on Windows from https://github.com/vim/vim-win32-installer
• Install fittencode.vim
• Open gvim and drag the file C:\Hello to the gvim window
• Press the shortcut key <c-l>
• Server returns {'detail': [{'ctx': {'error': 'Invalid \escape'}, 'type': 'json_invalid', 'msg': 'JSON decode error', 'input': {}, 'loc': ['body', 75]}]}

Solution

• Convert backslashes to slashes in the filename
• Check if the completion_data contains generated_text fields

Also Fix #4

[lzhengning]

Thank you for your contribution!