Scripts: File and Folder exists checks the kind, encode keeps slashes, every call is counted

- `File.encode` and `Folder.encode` leave `/` unescaped, as the File
  object reference lists it among the characters kept
  (https://extendscript.docsforadobe.dev/file-system-access/file-object/).
- `exists` is true for a `File` only when the path names something
  other than a folder, and for a `Folder` only when it names a folder.
- Fields of the records the app returns are read as own properties, so
  a property a script puts on `Object.prototype` does not turn a
  successful `open` into a failure.
- `File.encode`, `File.decode`, `File.isEncodingAvailable` and the
  `Folder` locations count as file calls and charge the host-call limit.
- On the web the locations are `null` and `File.tempName` names a file
  at the root, without asking the platform for a temporary folder.

docs/mcp.md

@@ -1819,6 +1819,8 @@ script reads files with them:
   objects for the same path are different objects.
 - `name`, `displayName`, `fsName`, `absoluteURI`, `fullName`, `relativeURI`, `path`, `parent` and `toString()` come
   from the path. `exists`, `length`, `created`, `modified`, `hidden`, `readonly` and `alias` read the file system.
+  `exists` is true for a `File` only when the path names something other than a folder, and for a `Folder` only
+  when it names a folder.
   `File.encode`, `File.decode` and `File.isEncodingAvailable` (and the same on `Folder`), `File.fs`, and the folders
   `Folder.temp`, `Folder.desktop`, `Folder.myDocuments`, `Folder.userData` and the others are there too.
 - `open('r')` opens a file to read, and `read([chars])`, `readln()`, `readch()`, `seek(pos[, mode])`, `tell()`, `eof`