Rework prefix parsing (closes #11)

[freya022]

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Description

Fixes a regex DoS vulnerability and unexpected exceptions on inputs like !!? where the prefix is !!, also reworks the message parsing and removes weird things

[Chew]

Thank you! Tested and found some issues:

Settings:

client.setPrefix("!");
client.setAlternativePrefix("?");
client.setPrefixes(new String[]{"<@!476489671313063936> ", "<", "bruh", "bruh, "});
client.setPrefixFunction(event -> {
    if (event.isFromGuild()) {
        return event.getGuild().getId();
    }
    return null;
});

(Valid prefixes are: !, ?. <@!476489671313063936> , <, bruh, bruh, , the server ID)

Tested with a simple ping command.

Input	event.getPrefix()	event.getArgs()
!ping	!	N/A
!ping bruh	!	"bruh"
?ping	?	N/A
?ping bruh	?	"bruh"
<@!476489671313063936> ping	<@!476489671313063936>	N/A
<@!476489671313063936> ping bruh	<@!476489671313063936>	"bruh"
<ping	<	N/A
<ping bruh	<	"bruh"
bruhping	bruh	N/A
bruhping bruh	bruh	"bruh"
bruh, ping	ping	N/A
bruh, ping bruh	ping	"bruh"
148195924567392257ping	148195924567392257	N/A
148195924567392257ping bruh	148195924567392257	"bruh"

[Chew]

Retested, gradle being dum, seems fine.