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/LibJS/Lexer.cpp

@@ -139,9 +139,10 @@ void Lexer::consume()
     auto did_reach_eof = [this] {
         if (m_position != m_source.length())
             return false;
+        m_eof = true;
+        m_current_char = '\0';
         m_position++;
         m_line_column++;
-        m_current_char = EOF;
         return true;
     };
 
@@ -276,7 +277,7 @@ bool Lexer::match(char a, char b, char c, char d) const
 
 bool Lexer::is_eof() const
 {
-    return m_current_char == EOF;
+    return m_eof;
 }
 
 bool Lexer::is_line_terminator() const
@@ -540,7 +541,7 @@ Token Lexer::next()
         } else {
             consume();
         }
-    } else if (m_current_char == EOF) {
+    } else if (m_eof) {
         if (unterminated_comment) {
             token_type = TokenType::Invalid;
             token_message = "Unterminated multi-line comment";