Make main program read a file

Just lex it and print the tokens, since that's the only thing we can
print atm. Run this in CI as a sanity check.

I was going to use clipp for cmdline parsing, but it's broken as of
C++20: muellan/clipp#53. It also seems to be
unmaintained, the PR that fixes it has been open for >2 years:
muellan/clipp#54

ci/build-and-test.sh

@@ -24,4 +24,4 @@ cmake -S $SRC_DIR -B $BUILD_DIR -DCMAKE_C_COMPILER=$C_COMPILER -DCMAKE_CXX_COMPI
 cmake --build $BUILD_DIR || exit $?
 
 ctest || exit $?
-$BUILD_DIR/bin/dumblang || exit $? # Just check that it runs
+$BUILD_DIR/bin/dumblang ../../../examples/example.dumb || exit $? # Just check that it runs

examples/example.dumb

@@ -0,0 +1,2 @@
+let x = 42
+let y = x + 3

src/dumblang/main.cpp

@@ -1,11 +1,37 @@
-#include "lexer.h"
+#include "parser.h"
 
+#include <filesystem>
+#include <fstream>
+#include <iostream>
 #include <sstream>
 
-int main()
+int main(int argc, char *argv[])
 {
-    std::stringstream ss1{"let x = 0\nlet y = 1"};
-    lexer::print_tokens(lexer::lex(ss1));
-    std::stringstream ss2;
-    lexer::print_tokens(lexer::lex(ss2));
+    // TODO Use CLI library. Clipp is unmaintained, find another one
+    if (argc != 2)
+    {
+        std::cerr << "USAGE: " << argv[0] << "<input file" << std::endl;
+        return 1;
+    }
+
+    std::filesystem::path infile{argv[1]};
+    std::cout << "Input file: " << infile << std::endl;
+
+    if (!std::filesystem::exists(infile))
+    {
+        std::cerr << "Input file " << infile << " does not exist";
+        return 1;
+    }
+
+    std::ifstream ifs;
+    ifs.open(infile, std::ifstream::in);
+    if (!ifs.is_open())
+    {
+        std::cerr << "Failed to open " << infile << std::endl;
+        return 1;
+    }
+
+    std::cout << "-----\n";
+    lexer::print_tokens(lexer::lex(ifs));
+    std::cout << "\n-----\n";
 }