Scripts: #include and the other preprocessor lines run, and $.evalFile runs a file

Before a script runs, `script.run` reads its preprocessor lines (`#` or
`//@` at the start of a line, outside strings and comments):

- `#include` and `//@include` put another file's text in place of the
  line, looked for beside the including file, then in each
  `#includepath` folder, then in each `JSINCLUDE` folder. A later
  `#includepath` replaces the list.
- `#script` names the run unless `name` is given; `#target` takes any
  value; `#strict off` is taken. `#strict on`, `#targetengine` and
  `#engine` are refused as unsupported, and any other `#name` as
  unknown.

`$.evalFile(path, timeout?)` runs a file in the same global environment
and returns its last value, `$.fileName` is the file being run and
`$.includePath` the include folders.

A script run from a file reads these files only in its folder and the
folders below it. Paths with a `..` name, links or junctions that lead
out, device names, streams and network paths are refused; a script
given as text reads none. A run reads at most 64 files 16 levels deep,
32 MiB of source in all and 1,024 paths looked up. An error in an
included or evaluated file is reported at that file's line and column,
and the report's new `file` field names it.

Docs: docs/mcp.md (script.run, Script Console), docs/target-app-parity.md.

crates/js/src/bytecode.rs

@@ -171,6 +171,8 @@ pub(crate) struct Handler {
 pub(crate) struct Source {
     pub(crate) lines: LineTable,
     pub(crate) text: Rc<[u16]>,
+    /// Which source this is, as [`crate::Realm::error_source`] gives it.
+    pub(crate) tag: u32,
 }
 
 #[derive(Debug)]