fix(lib/cli): relative paths, empty slides, and tests (#223)

* fix(lib/cli): relative paths, empty slides, and tests

This fixes two issues:

1. Empty slides are now reported as error, to prevent indexing error;
2. Changing the folder path will now produce an absolute path to slides, which was not the case before and would lead to a "file does not exist error".

A few tests were also added to cover those

* fix(lib): fix from_file, remove useless field, and more

* chore(tests): remove print

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

tests/test_main.py

@@ -0,0 +1,93 @@
+from pathlib import Path
+
+from click.testing import CliRunner
+
+from manim_slides.__main__ import cli
+
+
+def test_help() -> None:
+    runner = CliRunner()
+    results = runner.invoke(cli, ["-S", "--help"])
+
+    assert results.exit_code == 0
+
+    results = runner.invoke(cli, ["-S", "-h"])
+
+    assert results.exit_code == 0
+
+
+def test_defaults_to_present(folder_path: Path) -> None:
+    runner = CliRunner()
+
+    with runner.isolated_filesystem():
+        results = runner.invoke(
+            cli, ["BasicExample", "--folder", str(folder_path), "-s"]
+        )
+
+        assert results.exit_code == 0
+
+
+def test_present(folder_path: Path) -> None:
+    runner = CliRunner()
+
+    with runner.isolated_filesystem():
+        results = runner.invoke(
+            cli, ["present", "BasicExample", "--folder", str(folder_path), "-s"]
+        )
+
+        assert results.exit_code == 0
+
+
+def test_convert(folder_path: Path) -> None:
+    runner = CliRunner()
+
+    with runner.isolated_filesystem():
+        results = runner.invoke(
+            cli,
+            [
+                "convert",
+                "BasicExample",
+                "basic_example.html",
+                "--folder",
+                str(folder_path),
+            ],
+        )
+
+        assert results.exit_code == 0
+
+
+def test_init() -> None:
+    runner = CliRunner()
+
+    with runner.isolated_filesystem():
+        results = runner.invoke(
+            cli,
+            [
+                "init",
+                "--force",
+            ],
+        )
+
+        assert results.exit_code == 0
+
+
+def test_list_scenes(folder_path: Path) -> None:
+    runner = CliRunner()
+
+    with runner.isolated_filesystem():
+        results = runner.invoke(
+            cli,
+            [
+                "list-scenes",
+                "--folder",
+                str(folder_path),
+            ],
+        )
+
+        assert results.exit_code == 0
+        assert "BasicExample" in results.output
+
+
+def test_wizard() -> None:
+    # TODO
+    pass