fix(libexpr/lexer): fix flex warning about default rule

We were getting this flex lexer warning during build:
```
../src/libexpr/lexer.l:333: warning, -s option given but default rule can be matched
```

The lexer uses `%option nodefault` but the `PATH_START` state only had
rules for specific patterns (`PATH_SEG` and `HPATH_START`) without a
catch-all rule to handle unexpected input.

Added a catch-all rule with `unreachable()`. This code path should never
be reached in normal operation since `PATH_START` is only entered after
matching `PATH_SEG` or `HPATH_START`, and we immediately rewind to
re-parse those same patterns. The catch-all exists solely to satisfy
flex's `%option nodefault` requirement.

Author: lovesegfault

src/libexpr/lexer.l

@@ -243,6 +243,14 @@ or          { return OR_KW; }
   return HPATH;
 }
 
+<PATH_START>{ANY} |
+<PATH_START><<EOF>> {
+  /* This should be unreachable: PATH_START is only entered after matching
+     PATH_SEG or HPATH_START, and we rewind to re-parse those same patterns.
+     This rule exists to satisfy flex's %option nodefault requirement. */
+  unreachable();
+}
+
 {PATH} {
   if (yytext[yyleng-1] == '/')
     PUSH_STATE(INPATH_SLASH);