build: rename Exercise.baseName to name

The name "baseName" is confusing, since it refers to the UNIX basename
command, and not to the basename function in some programming languages
including Zig.

Use the std.fs.path.stem function to remove the file extension, instead
of slicing.

Remove the use of the assertion, since it is no longer necessary.
Instead, add a check to ensure that the exercise must be a Zig source
file in the validate_exercises function.

Update the validate_exercises function to check the last exercise, too.

test/tests.zig

@@ -323,7 +323,7 @@ fn heal(allocator: Allocator, exercises: []const Exercise, outdir: []const u8) !
     const patches_path = "patches/patches";
 
     for (exercises) |ex| {
-        const name = ex.baseName();
+        const name = ex.name();
 
         // Use the POSIX patch variant.
         const file = try join(allocator, &.{ exercises_path, ex.main_file });