[formatter] implement 'C' format specifier (#5647)

to apply a conversion after ':' or
to apply multiple conversions

for example {tags:CSl} or {tags:J - /Cl}
to convert list to string and lowercase it

docs/formatting.md

@@ -192,6 +192,12 @@ Format specifiers can be used for advanced formatting by using the options provi
     <td><code>{foo:Ro/()/}</code></td>
     <td><code>F()()&nbsp;Bar</code></td>
 </tr>
+<tr>
+    <td><code>C&lt;conversion(s)&gt;/</code></td>
+    <td>Apply <a href="#conversions">Conversions</a> to the current value</td>
+    <td><code>{tags:CSgc/}</code></td>
+    <td><code>"Sun-tree-water"</code></td>
+</tr>
 <tr>
     <td><code>S&lt;order&gt;/</code></td>
     <td>Sort a list. <code>&lt;order&gt;</code> can be either <strong>a</strong>scending or <strong>d</strong>escending/<strong>r</strong>everse. (default: <strong>a</strong>)</td>