[html] Fix indentation when entering a newline while in the middle of a tag

Example:

```
        <tag>|</tag>
```
Then press enter

Old behaviour:
```
        <tag>
            </tag>
```
New behaviour:
```
        <tag>
        </tag>
```

I also added a test for it

rc/filetype/html.kak

@@ -80,8 +80,8 @@ define-command -hidden html-indent-on-new-line %{
         try %{ execute-keys -draft <semicolon> K <a-&> }
         # filter previous line
         try %{ execute-keys -draft k : html-trim-indent <ret> }
-        # indent after lines ending with opening tag
+        # indent after lines ending with opening tag except when it starts with a closing tag
-        try %{ execute-keys -draft k <a-x> <a-k> <lt>(?!area)(?!base)(?!br)(?!col)(?!command)(?!embed)(?!hr)(?!img)(?!input)(?!keygen)(?!link)(?!menuitem)(?!meta)(?!param)(?!source)(?!track)(?!wbr)(?!/)(?!>)[a-zA-Z0-9_-]+[^>]*?>$ <ret> j <a-gt> } }
+        try %{ execute-keys -draft k <a-x> <a-k> <lt>(?!area)(?!base)(?!br)(?!col)(?!command)(?!embed)(?!hr)(?!img)(?!input)(?!keygen)(?!link)(?!menuitem)(?!meta)(?!param)(?!source)(?!track)(?!wbr)(?!/)(?!>)[a-zA-Z0-9_-]+[^>]*?>$ <ret>j<a-x><a-K>^\s*<lt>/<ret><a-gt> } }
 }
 
 ]

test/indent/html/indent-on-new-line/cmd

@@ -0,0 +1 @@
+i<ret><esc>

test/indent/html/indent-on-new-line/in

@@ -0,0 +1,3 @@
+<div>
+    <div>%( )</div>
+</div>

test/indent/html/indent-on-new-line/out

@@ -0,0 +1,4 @@
+<div>
+    <div>
+    </div>
+</div>

test/indent/html/indent-on-new-line/rc

@@ -0,0 +1,3 @@
+source "%val{runtime}/colors/default.kak"
+source "%val{runtime}/rc/filetype/html.kak"
+set buffer filetype html