Updated shebang & extention regex to be safer & match valid shellcheck shells

* `sh`
* `ash`
* `dash`
* `bash`
* `bats`
* `ksh`
[src](https://github.com/koalaman/shellcheck/blob/c2a15ce8e906ae6a12bcbe32eac7ac586fbcb59b/src/ShellCheck/Parser.hs#L3186-L3193)

---

Shebang regex:
Was: `'^#!.*sh'`
Now: `'^#!\(/\|/.*/\|/.* \)\(\(ba\|da\|k\|a\)*sh\|bats\)$'`

Test cases:
https://pastebin.com/BKB92wx2

Previous regex matched a lot of invalid shebangs & missed the 6 valid `bats` shebangs.

Current regex matches just the 6 correct shebangs from each valid shell.
There are probably some cases that I can't think of but it's a lot safer than the existing one.
A compromise would be something like: `'^#!/.*\(\(ba\|da\|k\|a\)*sh\|bats\)$'`
It matches the `zsh` `fish` and extra characters + valid shell ones but they're all at least valid.

---

File ending regex:
Was: `\.sh$|bash$`
Now: `.+\.(sh|bash|dash|ksh|ash|bats)$`

Test cases:
https://pastebin.com/JRJg8N13

Testing script:
```
test() {
	if [[ "$1" =~ .+\.(sh|bash|dash|ksh|ash|bats)$ ]]; then
		echo "match     - $1"
	else
		echo "not match - $1"
	fi
}

while IFS= read -r FILE; do
	if [[ "$FILE" != "" ]]; then
		test "$FILE"
	fi
done <<EOF
TEST CASES HERE
EOF
```

Previous regex would match `.sh` `bash` and miss `file.ksh` `file.dash` `file.ash` `file.bats`

Current regex only matches the 3 normal file names + also matches just `.sh` `.bash` etc, which are very unusual but valid file names.
There's no extra checking for slashes etc because file names & escapes could confuse things, just the extension should be enough.

Author: 06kellyjac

hooks/shellcheck.sh

@@ -36,11 +36,12 @@ parse_arguments() {
 parse_arguments "$@"
 
 for FILE in $files; do
-	if (head -1 "$FILE" | grep '^#!.*sh' >/dev/null); then
+	SHEBANG_REGEX='^#!\(/\|/.*/\|/.* \)\(\(ba\|da\|k\|a\)*sh\|bats\)$'
+	if (head -1 "$FILE" | grep "$SHEBANG_REGEX" >/dev/null); then
 		if ! shellcheck ${enable_list:+ --enable="$enable_list"} "$FILE"; then
 			exit_status=1
 		fi
-	elif [[ "$FILE" =~ \.sh$|bash$ ]]; then
+	elif [[ "$FILE" =~ .+\.(sh|bash|dash|ksh|ash|bats)$ ]]; then
 		echo "$FILE: missing shebang"
 		exit_status=1
 	fi