Map Unknown script to Zzzz short name

This is specified in UAX#24 (https://www.unicode.org/reports/tr24/#Relation_To_ISO15924)
And matches HarfBuzz's definition https://github.com/harfbuzz/harfbuzz/blob/0e6e18e74f31f4f3fd3817086b03490a24714888/src/hb-script-list.h#L240

Author: nicoburns

scripts/unicode.py

@@ -297,7 +297,7 @@ def generate_script_impl(f):
     #[inline]
     pub(crate) fn inner_short_name(self) -> &'static str {
         match self {
-            Script::Unknown => "",
+            Script::Unknown => "Zzzz",
             Script::Common => "Zyyy",
             Script::Inherited => "Zinh",
 """)
@@ -312,6 +312,7 @@ def generate_script_impl(f):
     #[inline]
     pub(crate) fn inner_from_short_name(input: &str) -> Option<Self> {
         match input {
+            "Zzzz" => Some(Script::Unknown),
             "Zyyy" => Some(Script::Common),
             "Zinh" => Some(Script::Inherited),
 """)

src/tables.rs

@@ -1598,7 +1598,7 @@ impl Script {
     #[inline]
     pub(crate) fn inner_short_name(self) -> &'static str {
         match self {
-            Script::Unknown => "",
+            Script::Unknown => "Zzzz",
             Script::Common => "Zyyy",
             Script::Inherited => "Zinh",
             Script::Adlam => "Adlm",
@@ -1775,6 +1775,7 @@ impl Script {
     #[inline]
     pub(crate) fn inner_from_short_name(input: &str) -> Option<Self> {
         match input {
+            "Zzzz" => Some(Script::Unknown),
             "Zyyy" => Some(Script::Common),
             "Zinh" => Some(Script::Inherited),
             "Adlm" => Some(Script::Adlam),