feat: add new syntax {key} for pug (#259)

Author: Misha Zamkevich

docs/preprocessing.md

@@ -297,7 +297,7 @@ 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()`, `+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()`, `+debug()`.
 
 ```pug
 ul

src/transformers/pug.ts

@@ -18,6 +18,11 @@ mixin elseif(condition)
 %_| {:else if !{condition}}
 %_block
 
+mixin key(expression)
+%_| {#key !{expression}}
+%_block
+%_| {/key}
+
 mixin each(loop)
 %_| {#each !{loop}}
 %_block