fix: add pug const mixin (#518)

Closes #467

Author: fedorovvvv

Diff for: docs/preprocessing.md

@@ -290,7 +290,7 @@ Apart from those, the Pug preprocessor accepts:
 
 **Template blocks:**
 
-Some of Svelte's template syntax is invalid in Pug. `svelte-preprocess` provides some pug mixins to represent svelte's `{#...}{/...}` blocks: `+if()`, `+else()`, `+elseif()`, `+each()`, `+key()`, `+await()`, `+then()`, `+catch()`, `+html()`, `+debug()`.
+Some of Svelte's template syntax is invalid in Pug. `svelte-preprocess` provides some pug mixins to represent svelte's `{#...}{/...}` blocks: `+if()`, `+else()`, `+elseif()`, `+each()`, `+key()`, `+await()`, `+then()`, `+catch()`, `+html()`, `+const()`, `+debug()`.
 
 ```pug
 ul

Diff for: src/transformers/pug.ts

@@ -44,6 +44,9 @@ mixin catch(error)
 mixin html(expression)
 %_| {@html !{expression}}
 
+mixin const(expression)
+%_| {@const !{expression}}
+
 mixin debug(variables)
 %_| {@debug !{variables}}`.replace(
     /%_/g,