fix(exts/devin-lang): Allow code blocks to start without content #101

The DevInParser.bnf has been modified to support code blocks that start with a CODE_BLOCK_START token without requiring mandatory content. This change ensures that code blocks are not incorrectly parsed when they are intended to be empty.

Two test cases have been added to DevInParsingTest.kt to validate the parser's behavior with both empty and non-empty code blocks. The testData directory has been updated with a new file, BlockStartOnly.devin, which contains a code block starting with CODE_BLOCK_START without any content, illustrating a valid usage scenario.

This commit adheres to the Conventional Commits specification for fixes related to the DevInLanguage extension.

Author: phodal

exts/devin-lang/src/test/testData/parser/BlockStartOnly.txt

@@ -0,0 +1,5 @@
+DevInFile
+  CodeBlockElement(CODE)
+    PsiElement(DevInTokenType.CODE_BLOCK_START)('```')
+    ASTWrapperPsiElement(CODE_CONTENTS)
+      <empty list>