Move linters into seperate module.

Give linters an abstract type to inherit, since they share an interface.
Add empty README

printer-linter/src/printerlinter/linters/meshes.py

@@ -0,0 +1,42 @@
+from typing import Iterator
+
+from ..diagnostic import Diagnostic
+
+
+class Meshes:
+    def __init__(self, file, settings) -> None:
+        super().__init__(file, settings)
+        self._max_file_size = self._settings.get("diagnostic-mesh-file-size", 1e6)
+
+    def check(self) -> Iterator[Diagnostic]:
+        if self._settings["checks"].get("diagnostic-mesh-file-extension", False):
+            for check in self.checkFileFormat():
+                yield check
+
+        if self._settings["checks"].get("diagnostic-mesh-file-size", False):
+            for check in self.checkFileSize():
+                yield check
+
+        yield
+
+    def checkFileFormat(self) -> None:
+        if self._file.suffix.lower() not in (".3mf", ".obj", ".stl"):
+            yield Diagnostic(
+                file = self._file,
+                diagnostic_name = "diagnostic-mesh-file-extension",
+                message = f"Extension {self._file.suffix} not supported, use 3mf, obj or stl",
+                level = "Error",
+                offset = 1
+            )
+        yield
+
+    def checkFileSize(self):
+        if self._file.stat().st_size > self._max_file_size:
+            yield Diagnostic(
+                file = self._file,
+                diagnostic_name = "diagnostic-mesh-file-size",
+                message = f"Mesh file with a size {self._file.stat().st_size} is bigger then allowed maximum of {self._max_file_size}",
+                level = "Error",
+                offset = 1
+            )
+        yield