Namespaced identifier improvements

[SpaceWalkerRS]

Adds new functionality to NamespacedIdentifier and new utility methods to NamespacedIdentifiers.

NamespacedIdentifier

New methods:

• separator(): returns the char used as the separator in the String-representation of the namespaced identifier. This separator character is also used to check equality.
• namespaced(String): returns a copy of the namespaced identifier with the given namespace.
• subidentifier(int) and subidentifier(int, int): similar to String's substring methods; returns a copy of the namespaced identifier where the identifier is a substring of the orignal identifier.

Examples:

NamespacedIdentifier identifier = NamespacedIdentifiers.from("example", "recipes/cookie.json");

identifier.separator(); // returns ':', String-representation: example:recipes/cookie.json
identifier = identifier.namespaced("minecraft"); // -> minecraft:recipes/cookie.json
identifier = identifier.subidentifier(8); // -> minecraft:cookie.json
identifier = identifier.subidentifier(6, 11); // -> minecraft:cookie

NamespacedIdentifiers

New methods:

• isValid(NamespacedIdentifier): returns whether the given namespaced identifier conforms to spec (only chars a-z0-9-._/ allowed).
• isValidNamespace(String): returns whether the given String is a valid namespace.
• isValidIdentifier(String): returns whether the given String is a valid identifier.
• isCharAllowed(char): returns whether the given char is allowed in the String representation of a namespaced identifier.

Other changes:

• equals(NamespacedIdentifier, NamespacedIdentifier) now compares the separator() chars in addition to the namespace and identifier Strings.