feat(LEXER): support function names containing the at sign

[niall-byrne]

This is almost a repeat of the last commit.

Bash also supports having the at sign in function names:

@fake.python.decorator.fn() { echo "hello there"; }

This short commit expands the regex to support this usage.

---

I added a second commit to handle multiline subshells by ignoring the subshell boundaries:

(
   echo "I'm stuck in a subshell, let me out."
)