feat: support escaping @ in the comment

[sigoden]

If we use # @ but @.. is not a valid comment tag, argc will throw an error.

# @describe Patch utils
# 
# Here is an example of a patch block:
# --- a/hello.py
# +++ b/hello.py
# @@ ... @@
#  def hello():
# -    print("Hello World")
# +    name = input("What is your name? ")
# +    print(f"Hello {name}")

eval "$(argc --argc-eval "$0" "$@")"

@@(line 8) is unknown tag

This PR allow escaping @ with \ to avoid such error.

-# @@ ... @@
+# \@@ ... @@