Specify the UTI types for files in the Info.plist

This sets the LSItemContentTypes key to ['public.source-code'], a
declared text UTI [1], which should permit QuickLook previews.

Fixes Microsoft/vscode#25843

[1]
https://developer.apple.com/library/content/documentation/Miscellaneous/Reference/UTIRef/Articles/System-DeclaredUniformTypeIdentifiers.html#//apple_ref/doc/uid/TP40009259-SW1

build/gulpfile.vscode.js

@@ -138,6 +138,7 @@ const config = {
 		role: 'Editor',
 		ostypes: ["TEXT", "utxt", "TUTX", "****"],
 		extensions: ["ascx", "asp", "aspx", "bash", "bash_login", "bash_logout", "bash_profile", "bashrc", "bat", "bowerrc", "c", "cc", "clj", "cljs", "cljx", "clojure", "cmd", "code-workspace", "coffee", "config", "cpp", "cs", "cshtml", "csproj", "css", "csx", "ctp", "cxx", "dockerfile", "dot", "dtd", "editorconfig", "edn", "eyaml", "eyml", "fs", "fsi", "fsscript", "fsx", "gemspec", "gitattributes", "gitconfig", "gitignore", "go", "h", "handlebars", "hbs", "hh", "hpp", "htm", "html", "hxx", "ini", "jade", "jav", "java", "js", "jscsrc", "jshintrc", "jshtm", "json", "jsp", "less", "lua", "m", "makefile", "markdown", "md", "mdoc", "mdown", "mdtext", "mdtxt", "mdwn", "mkd", "mkdn", "ml", "mli", "php", "phtml", "pl", "pl6", "pm", "pm6", "pod", "pp", "profile", "properties", "ps1", "psd1", "psgi", "psm1", "py", "r", "rb", "rhistory", "rprofile", "rs", "rt", "scss", "sh", "shtml", "sql", "svg", "svgz", "t", "ts", "txt", "vb", "wxi", "wxl", "wxs", "xaml", "xcodeproj", "xcworkspace", "xml", "yaml", "yml", "zlogin", "zlogout", "zprofile", "zsh", "zshenv", "zshrc"],
+		utis: ['public.source-code'],
 		iconFile: 'resources/darwin/code_file.icns'
 	}],
 	darwinBundleURLTypes: [{