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.h

@@ -46,6 +46,7 @@ private:
     size_t m_position { 0 };
     Token m_current_token;
     char m_current_char { 0 };
+    bool m_eof { false };
 
     StringView m_filename;
     size_t m_line_number { 1 };