Allow new file and edit file preview if it has editable extension (#23624)

Close #23579 
Inspired by
[idea](https://github.com/go-gitea/gitea/issues/23579#issuecomment-1475429247)
from @brechtvl
In this PR, the behavior is when extension switches from writatble to
not, preview will hide, and vice versa.

demo:

https://user-images.githubusercontent.com/17645053/226786119-d20063da-8763-41ce-9b00-ae34929120e1.mov

---------

Co-authored-by: silverwind <me@silverwind.io>

templates/repo/editor/edit.tmpl

@@ -30,8 +30,8 @@
 			<div class="field">
 				<div class="ui top attached tabular menu" data-write="write" data-preview="preview" data-diff="diff">
 					<a class="active item" data-tab="write">{{svg "octicon-code"}} {{if .IsNewFile}}{{.locale.Tr "repo.editor.new_file"}}{{else}}{{.locale.Tr "repo.editor.edit_file"}}{{end}}</a>
-					{{if not .IsNewFile}}
 					<a class="item" data-tab="preview" data-url="{{.Repository.Link}}/markup" data-context="{{.RepoLink}}/src/{{.BranchNameSubURL}}" data-markup-mode="file">{{svg "octicon-eye"}} {{.locale.Tr "preview"}}</a>
+					{{if not .IsNewFile}}
 					<a class="item" data-tab="diff" data-url="{{.RepoLink}}/_preview/{{.BranchName | PathEscapeSegments}}/{{.TreePath | PathEscapeSegments}}" data-context="{{.BranchLink}}">{{svg "octicon-diff"}} {{.locale.Tr "repo.editor.preview_changes"}}</a>
 					{{end}}
 				</div>