fix: image path issue with msys2/vim (iamcco#604)

* fix: image path issue with msys2/vim

1) Issue background
    a) on 'mingw32' terminal
    b) executable 'vim' in /usr/bin
    c) executable 'node' in /mingw32/bin.
    d) plugin 'markdow-preview.nvim' in ~/.vim/plugged
    e) app/route.js:68 fileDir is a  unix path
    f) image missing in browser preview
2) Issue fixing
    add code to app/route.js:
    a) extract info from environment variable MINGW_HOME
    b) prefixing imgPath with sth. like "E:\msys64\"
3) No issue alternative
    a) on 'ucrt64' terminal
    b) executable 'nvim' in /ucrt64/bin
    c) executable 'node' in /ucrt64/bin
    d) plugin 'markdown-preview.nvim' in ~/.vim/plugged
    e) app/route.js:68 fileDir is a win path
    f) image ok in browser preview

* fix: now work on any windows disk device

1) Based on info from MINGW_HOME is error-prone. Maybe the document is
   located on a windisk other than that of msys2 installation.
2) use 'cygpath.exe' to convert unix-like 'fileDir' to win-like one.
3) the conversion operation must follow the first assignment of 'fileDir'.

app/routes.js

@@ -65,8 +65,26 @@ use(async (req, res, next) => {
     const buffers = await plugin.nvim.buffers
     const buffer = buffers.find(b => b.id === Number(req.bufnr))
     if (buffer) {
-      const fileDir = await plugin.nvim.call('expand', `#${req.bufnr}:p:h`)
+      let fileDir = await plugin.nvim.call('expand', `#${req.bufnr}:p:h`)
       logger.info('fileDir', fileDir)
+
+      const  mingw_home=process.env.MINGW_HOME;
+      if (mingw_home){
+        if(! fileDir.includes(':')){
+          // fileDir is unix-like:      /Z/x/y/...., 'Z' means Z:
+          // the win-like fileDir should be: Z:\x\y...
+          const cygpath = 'cygpath.exe'
+          const cmd=cygpath+' -w'+' -a '+fileDir ;
+          logger.info('cmd',cmd)
+
+          const { execSync } = require('node:child_process');
+          const result = execSync(cmd);
+          fileDir=result.toString('utf8').replace('\n','');
+
+          logger.info('New fileDir',fileDir);
+        }  
+      }
+
       let imgPath = decodeURIComponent(decodeURIComponent(req.asPath.replace(reg, '')))
       imgPath = imgPath.replace(/\\ /g, ' ')
       if (imgPath[0] !== '/' && imgPath[0] !== '\\') {
@@ -82,7 +100,8 @@ use(async (req, res, next) => {
           }
         }
       }
-      logger.info('imgPath', imgPath)
+      logger.info('imgPath', imgPath);
+
       if (fs.existsSync(imgPath) && !fs.statSync(imgPath).isDirectory()) {
         if (imgPath.endsWith('svg')) {
           res.setHeader('content-type', 'image/svg+xml')