Handle zero-width space in text markup.

org_mode_samples/object/text_markup/verbatim_with_equals_inside.org

@@ -1 +1,4 @@
 foo ==>bar=.
+
+# This uses a zero-width space to escape the equals signs to make the verbatim not end.
+=lorem =​= ipsum=

src/parser/text_markup.rs

@@ -231,7 +231,8 @@ fn _text_markup_string<'b, 'g, 'r, 's, 'c>(
 ) -> Res<OrgSource<'s>, OrgSource<'s>> {
     let (remaining, _) = pre(context, input)?;
     let (remaining, open) = tag(marker_symbol)(remaining)?;
-    let (remaining, _peek_not_whitespace) = peek(not(multispace1))(remaining)?;
+    let (remaining, _peek_not_whitespace) =
+        peek(verify(anychar, |c| !c.is_whitespace() && *c != '\u{200B}'))(remaining)?;
     let text_markup_end_specialized = text_markup_end(open.into());
     let parser_context = ContextElement::ExitMatcherNode(ExitMatcherNode {
         class: ExitClass::Gamma,

src/parser/util.rs

@@ -127,15 +127,14 @@ pub fn start_of_line<'s>(input: OrgSource<'s>) -> Res<OrgSource<'s>, ()> {
 #[cfg_attr(feature = "tracing", tracing::instrument(ret, level = "debug"))]
 pub fn preceded_by_whitespace<'s>(input: OrgSource<'s>) -> Res<OrgSource<'s>, ()> {
     let preceding_character = input.get_preceding_character();
-    match preceding_character {
-        Some('\n') | Some('\r') | Some(' ') | Some('\t') => {}
-        // If None, we are at the start of the file which is not allowed
-        None | Some(_) => {
+    if !preceding_character
+        .map(|c| c.is_whitespace() || c == '\u{200B}') // 200B = Zero-width space
+        .unwrap_or(false)
+    {
         return Err(nom::Err::Error(CustomError::MyError(MyError(
             "Not preceded by whitespace.".into(),
         ))));
     }
-    };
     Ok((input, ()))
 }