Add packages/strings/strings.md

packages/strings/strings.md

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+# Strings library
+
+This library adds extra string manipulation functions to ground. It is entirely coded in ground.
+
+Please note that you cannot copy a single function from the package, as some functions rely on other functions in the library.
+
+## Functions
+
+### fun -char !findChar -string &str -char &char
+
+Finds the index of $char in $string
+
+### fun -char !findCharInstance -string &str -char &char -int &instance
+
+Finds the index of the `$instance`th instance of $char in $str
+
+### fun -string !mid -string &str -int &idx -int &len
+
+Finds $len characters of $str, starting at $idx.
+
+### fun -string !inBrackets -string &str -char &left -char &right -int &instance
+
+Finds the content of the `$instance`th instance of brackets, with the left bracket being the $left char and the right bracket of $right char.
+
+### fun -string !matchStart -string &str -string &instr
+
+Checks if the start of $str matches $instr.