refactor(fluent-pseudo): Use once_cell to avoid unsafe mutable static without bumping MSRV

This can be updated to use the *std* implementation once our MSRV can be
1.70 or newer. For now this conservatively maintains the lower MSRV
while dodging the undefined behavior of a mutable reference to a mutable
static. Technically what we were doing would work, but relying on
undefined behavior that we were only bending now breaking isn't a great
practice. Two less unsafe blocks.

Note the once_cell 1.21.x release series requires 1.66.0, which is
a bump up from what fluent-pseudo currently needs.

Author: alerque

fluent-pseudo/Cargo.toml

@@ -25,3 +25,4 @@ include = [
 
 [dependencies]
 regex = "1.9"
+once_cell = "1.20"

fluent-pseudo/src/lib.rs

@@ -1,3 +1,4 @@
+use once_cell::sync::Lazy;
 use regex::Captures;
 use regex::Regex;
 use std::borrow::Cow;
@@ -20,8 +21,8 @@ static FLIPPED_CAPS_MAP: &[char] = &[
     '⊥', '∩', 'Ʌ', 'M', 'X', '⅄', 'Z',
 ];
 
-static mut RE_EXCLUDED: Option<Regex> = None;
-static mut RE_AZ: Option<Regex> = None;
+static RE_EXCLUDED: Lazy<Regex> = Lazy::new(|| Regex::new(r"&[#\w]+;|<\s*.+?\s*>").unwrap());
+static RE_AZ: Lazy<Regex> = Lazy::new(|| Regex::new(r"[a-zA-Z]").unwrap());
 
 pub fn transform_dom(s: &str, flipped: bool, elongate: bool, with_markers: bool) -> Cow<str> {
     // Exclude access-keys and other single-char messages
@@ -30,15 +31,12 @@ pub fn transform_dom(s: &str, flipped: bool, elongate: bool, with_markers: bool)
     }
 
     // XML entities (&#x202a;) and XML tags.
-    let re_excluded =
-        unsafe { RE_EXCLUDED.get_or_insert_with(|| Regex::new(r"&[#\w]+;|<\s*.+?\s*>").unwrap()) };
-
     let mut result = Cow::from(s);
 
     let mut pos = 0;
     let mut diff = 0;
 
-    for cap in re_excluded.captures_iter(s) {
+    for cap in RE_EXCLUDED.captures_iter(s) {
         let capture = cap.get(0).unwrap();
 
         let sub_len = capture.start() - pos;
@@ -65,15 +63,13 @@ pub fn transform_dom(s: &str, flipped: bool, elongate: bool, with_markers: bool)
 }
 
 pub fn transform(s: &str, flipped: bool, elongate: bool) -> Cow<str> {
-    let re_az = unsafe { RE_AZ.get_or_insert_with(|| Regex::new(r"[a-zA-Z]").unwrap()) };
-
     let (small_map, caps_map) = if flipped {
         (FLIPPED_SMALL_MAP, FLIPPED_CAPS_MAP)
     } else {
         (TRANSFORM_SMALL_MAP, TRANSFORM_CAPS_MAP)
     };
 
-    re_az.replace_all(s, |caps: &Captures| {
+    RE_AZ.replace_all(s, |caps: &Captures| {
         let ch = caps[0].chars().next().unwrap();
         let cc = ch as u8;
         if (97..=122).contains(&cc) {