Add 'doc' to TAGS_WITHOUT_MARKUP

reason: doc tag cannot have markup, so we strip it if present in stage 2.

Author: jamesmengo

packages/liquid-html-parser/src/grammar.ts

@@ -53,4 +53,5 @@ export const TAGS_WITHOUT_MARKUP = [
   'continue',
   'comment',
   'raw',
+  'doc',
 ];

packages/liquid-html-parser/src/stage-2-ast.spec.ts

@@ -1224,6 +1224,7 @@ describe('Unit: Stage 2 (AST)', () => {
       ast = toLiquidAST(`{% doc %}{% enddoc %}`);
       expectPath(ast, 'children.0.type').to.eql('LiquidRawTag');
       expectPath(ast, 'children.0.name').to.eql('doc');
+      expectPath(ast, 'children.0.markup').toEqual('');
       expectPath(ast, 'children.0.body.value').to.eql('');
       expectPath(ast, 'children.0.body.type').toEqual('RawMarkup');
       expectPath(ast, 'children.0.body.nodes').toEqual([]);