feat(furas): introduce a basic lexer

Closes: #56

furas/include/furas/lexer.hpp

@@ -0,0 +1,47 @@
+#ifndef FURAS_LEXER_HPP
+#define FURAS_LEXER_HPP
+
+#include "furas/token.hpp"
+#include "furlang/result.hpp"
+
+#include <cstddef>
+#include <string_view>
+
+namespace furas {
+
+struct lexer_location {
+    std::string_view filename;
+    std::size_t      row, col;
+};
+
+struct lexer_error {
+    enum type {
+        EndOfFile = 0,
+        UnknownCharacter,
+    } type;
+
+    lexer_location location;
+    std::string    message;
+};
+
+using token_r = furlang::result<token, lexer_error>;
+
+class lexer {
+public:
+    lexer(std::string_view filename, std::string_view content)
+      : m_filename(filename), m_content(content) {}
+
+    token_r next_token();
+private:
+    constexpr lexer_location location() const { return { m_filename, m_cursor - m_lineStart, m_column }; }
+private:
+    std::string_view m_filename;
+    std::string_view m_content;
+    std::size_t      m_cursor    = 0;
+    std::size_t      m_lineStart = 0;
+    std::size_t      m_column    = 0;
+};
+
+} // namespace furas
+
+#endif // FURAS_LEXER_HPP