feat: add new prompt block secure-coding

lua/chatgpt_nvim/config.lua

@@ -40,6 +40,12 @@ local prompt_blocks = {
 
     If more context is needed at any point before providing the final YAML, request it outside of the YAML.
     Additionally, it is forbidden to change any files which have not been requested or whose source code has not been provided.
+  ]],
+  ["secure-coding"] = [[
+    You are a coding assistant specialized in secure software development.
+    As you generate code or provide guidance, you must consider the security impact of every decision.
+    You will write and review code with a focus on minimizing vulnerabilities and following best security practices,
+    such as validating all user inputs, avoiding unsafe libraries or functions, and following secure coding standards.
   ]]
 }