feat(navbar): navbar link icon item

[ldez]

As the item has a parameter icon, it must be handled before icon only item.
To avoid repetition, I moved the code related to links into a dedicated partial.
I made a commit only for that; it can help the review.

The icon is supported for "normal" link and "menu" (parent/children) links.

Closes #359

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[imfing]

looks good. did some minor alignment adjustments.

[ldez]

Do you want me to add some icons to the example site?

[imfing]

Do you want me to add some icons to the example site?

sure, we can do this for one nested + one regular, too many icons might be overwhelming.

diff --git a/exampleSite/hugo.yaml b/exampleSite/hugo.yaml
index 79ecca0..e72889b 100644
--- a/exampleSite/hugo.yaml
+++ b/exampleSite/hugo.yaml
@@ -85,6 +85,9 @@ menu:
       name: Showcase
       pageRef: /showcase
       weight: 5
+      params:
+        icon: collection
+        type: link
     - name: Search
       weight: 6
       params:
@@ -98,6 +101,9 @@ menu:
       name: Development ↗
       url: https://imfing.github.io/hextra/versions/latest/
       parent: versions
+      params:
+        icon: beaker
+        type: link
     - identifier: v0.9
       name: v0.9 ↗
       url: https://imfing.github.io/hextra/versions/v0.9/