Userland: Allow building SerenityOS with -funsigned-char

Some of the code assumed that chars were always signed while that is
not the case on ARM hosts.

Also, some of the code tried to use EOF (-1) in a way similar to what
fgetc() does, however instead of storing the characters in an int
variable a char was used.

While this seemed to work it also meant that character 0xFF would be
incorrectly seen as an end-of-file.

Careful reading of fgetc() reveals that fgetc() stores character
data in an int where valid characters are in the range of 0-255 and
the EOF value is explicitly outside of that range (usually -1).

Userland/Libraries/LibRegex/RegexLexer.h

@@ -76,7 +76,7 @@ public:
     const auto& source() const { return m_source; }
 
 private:
-    ALWAYS_INLINE char peek(size_t offset = 0) const;
+    ALWAYS_INLINE int peek(size_t offset = 0) const;
     ALWAYS_INLINE void consume();
 
     StringView m_source {};