fix code inline normalize

[OpportunityLiu]

The old normalize renderer only use two `` for code inlines which contain `.

Add:

1. When n of continuous ` is included in the content, n+1 of ` will be used as delimiter.
2. When the content starts or ends with `, a space will be added to separate delimiter and content.
3. When the content is empty, a space will be used as content to prevent it be rendered to ``.

   a. `` 
   b. ` `
   

   a. ``
   b.

[OpportunityLiu]

We need a .gitattributes file

[xoofx]

Thanks!