Make Vim installation more standard and less intrusive (#1715)

* Better handling of existing Vim configuration

- Check for an existing vimrc file in (almost) all standard locations for Vim in Windows.
- Use _vimrc instead of .vimrc as default vimrc file name.

* Don't set VIM environment variable

vim.json

@@ -17,17 +17,14 @@
         "vim.exe",
         "gvim.exe"
     ],
-    "post_install": "if(!(test-path ~\\.vimrc)) {
-        cp \"$dir\\vimrc_example.vim\" ~\\.vimrc
-        \"set shell=$((gcm powershell).path)\\ -executionpolicy\\ bypass\" | out-file  ~\\.vimrc -append -encoding ascii
-        echo '~/.vimrc was created with your shell set to Powershell.'
-    } else { echo '~/.vimrc exists, skipping' }",
+    "post_install": "if( !(test-path ~\\.vimrc) -and !(test-path ~\\_vimrc) -and !(test-path ~\\vimfiles\\vimrc) -and !(test-path $env:VIM\\_vimrc) ) {
+        cp \"$dir\\vimrc_example.vim\" ~\\_vimrc
+        \"set shell=$((gcm powershell).path)\\ -executionpolicy\\ bypass\" | out-file  ~\\_vimrc -append -encoding ascii
+        echo 'Default vimrc file created in ~/_vimrc, with shell set to PowerShell.'
+    } else { echo 'Existing vimrc file found, no default configuration created' }",
     "checkver": {
         "github": "https://github.com/vim/vim-win32-installer"
     },
-    "env_set": {
-        "VIM": "$dir"
-    },
     "autoupdate": {
         "architecture": {
             "32bit": {