Compiler: Add early error for output path being a directory.

- Introduce new `Path` type and an array of build paths on the build context.
- Resolve input and output paths/files early (before parsing).
- Error early if inputs are missing or outputs are directories.
- Plumb new file path generation into linker stage instead of its adhoc method.

TODO:
- Remove more adhoc file path generation in parser and linker stage.
- Make intermediate object file generation use new path system.
- Round out and robustify Path helper functions.

tests/core/math/big/build.bat

@@ -4,7 +4,7 @@ set PATH_TO_ODIN==..\..\..\..\odin
 set TEST_ARGS=-fast-tests
 set TEST_ARGS=-no-random
 set TEST_ARGS=
-set OUT_NAME=math_big_test_library
+set OUT_NAME=math_big_test_library.dll
 set COMMON=-build-mode:shared -show-timings -no-bounds-check -define:MATH_BIG_EXE=false -vet -strict-style
 echo ---
 echo Running core:math/big tests