goalc/repl: cleanup of goalc/REPL code and some QoL improvements (#2104)

- lets you split up your `startup.gc` file into two sections
  - one that runs on initial startup / reloads
  - the other that runs when you listen to a target
- allows for customization of the keybinds added a month or so ago
- removes a useless flag (--startup-cmd) and marks others for
deprecation.
- added another help prompt that lists all the keybinds and what they do

Co-authored-by: water <awaterford111445@gmail.com>

common/util/string_util.cpp

@@ -2,6 +2,8 @@
 
 #include <regex>
 
+#include "common/util/diff.h"
+
 namespace str_util {
 
 const std::string WHITESPACE = " \n\r\t\f\v";
@@ -47,4 +49,12 @@ bool valid_regex(const std::string& regex) {
   }
   return true;
 }
+
+std::string diff(const std::string& lhs, const std::string& rhs) {
+  return google_diff::diff_strings(lhs, rhs);
+}
+/// Default splits on \n characters
+std::vector<std::string> split(const ::std::string& str, char delimiter) {
+  return google_diff::split_string(str, delimiter);
+}
 }  // namespace str_util