refactor(menu): use .Params.icon to configure menu item icon

Using [pre] field will result a warning, but will keeping working for compatibility

exampleSite/config.yaml

@@ -174,10 +174,10 @@ menu:
           name: Home
           url: /
           weight: -100
-          pre: home
           params:
               ### For demonstration purpose, the home link will be open in a new tab
               newTab: true
+              icon: home
 
 related:
     includeNewer: true

exampleSite/content/page/about.md

@@ -11,7 +11,8 @@ lastmod: '2020-10-09'
 menu:
     main: 
         weight: -90
-        pre: user
+        params:
+            icon: user
 ---
 
 Written in Go, Hugo is an open source static site generator available under the [Apache Licence 2.0.](https://github.com/gohugoio/hugo/blob/master/LICENSE) Hugo supports TOML, YAML and JSON data file types, Markdown and HTML content files and uses shortcodes to add rich content. Other notable features are taxonomies, multilingual mode, image processing, custom output formats, HTML/CSS/JS minification and support for Sass SCSS workflows.

exampleSite/content/page/archives.md

@@ -6,5 +6,6 @@ slug: "archives"
 menu:
     main:
         weight: -70
-        pre: archives
+        params: 
+            icon: archives
 ---

exampleSite/content/page/search.md

@@ -8,5 +8,6 @@ outputs:
 menu:
     main:
         weight: -60
-        pre: search
+        params: 
+            icon: search
 ---

layouts/partials/sidebar/left.html

@@ -40,8 +40,12 @@
 
         <li {{ if $active }} class='current' {{ end }}>
             <a href='{{ .URL | relLangURL }}' {{ if eq .Params.newTab true }}target="_blank"{{ end }}>
+                {{ $icon := default .Pre .Params.Icon }}
                 {{ if .Pre }}
-                    {{ partial "helper/icon" .Pre }}
+                    {{ warnf "Menu item [%s] is using [pre] field to set icon, please use [params.icon] instead" .URL }}
+                {{ end }}
+                {{ with $icon }}
+                    {{ partial "helper/icon" . }}
                 {{ end }}
                 <span>{{- .Name -}}</span>
             </a>