fix: add pug mixins to support svelte-5 syntax (#654)

* support for svelte-5 #snippet and @render

* Update preprocessing.md

docs/preprocessing.md

@@ -291,7 +291,7 @@ 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()`, `+const()`, `+debug()`.
+Some of Svelte's template syntax is invalid in Pug. `svelte-preprocess` provides some pug mixins to represent svelte's `{#...}{/...}` and `{@...}` blocks: `+if()`, `+else()`, `+elseif()`, `+each()`, `+key()`, `+await()`, `+then()`, `+catch()`, `+html()`, `+const()`, `+debug()`, `+snippet()`, `+render()`.
 
 ```pug
 ul

src/transformers/pug.ts

@@ -47,7 +47,15 @@ mixin const(expression)
 %_| {@const !{expression}}
 
 mixin debug(variables)
-%_| {@debug !{variables}}`.replace(
+%_| {@debug !{variables}}
+
+mixin snippet(expression)
+%_| {#snippet !{expression}}
+%_block
+%_| {/snippet}
+
+mixin render(expression)
+%_| {@render !{expression}}`.replace(
     /%_/g,
     indentationType === 'tab' ? '\t' : '  ',
   );