fix: hardcoded '/' in normalized options.fs, fixes #412, #408

Changes including:
- will not call fs.resolve when normalizing `fs`
- removed hardcoded `/`
- mandatory `fs.dirname` for relative reference
- mandatory `fs.sep`, defaults to '/'

src/render/expression.ts

@@ -21,7 +21,7 @@ export class Expression {
     this.postfix = [...toPostfix(tokens)]
   }
   public * evaluate (ctx: Context, lenient: boolean): any {
-    assert(ctx, () => 'unable to evaluate: context not defined')
+    assert(ctx, 'unable to evaluate: context not defined')
     const operands: any[] = []
     for (const token of this.postfix) {
       if (TypeGuards.isOperatorToken(token)) {