Add example for JS::Stencil

[arai-a]

as mentioned in #60 review comment, clone functionality is removed and that needs to be rewritten with JS::Stencil APIs.

I'll look into adding examples

[kulsoomzahra]

@arai-a Can you please point me to a good first issue or maybe somewhere that can be a nice
place to start for first-time contributors? Thank you so much!

[arai-a]

Hi, welcome.
This is also good-first-issue, in term of the complexity of the API.

APIs to cover in this example:

• Stencil APIs, listed in JSStencil.h:
  • JS::Stencil opaque struct, which represents the result of compilation
  • RefPtr class, which represents reference-counted pointer, used for JS::Stencil
  • JS::SourceText class, which holds the source text
  • JS::CompileOptions class, which represents the options for compilation
  • JS::CompileGlobalScriptToStencil function, which compiles given source text into JS::Stencil
  • JS::InstantiateGlobalStencil function, which converts JS::Stencil into JSScript structure (JSScript is GC-structure that represents either top-level script or function script)
• JS_ExecuteScript function, which executes JSScript and returns the evaluation result

similar unit test is stored in the SpiderMonkey source: testStencil.cpp

Here's the list of things to do:

Prerequisite:

• Clone this repository (esr91 branch) and build examples

This issue:

• Add a new source file in examples directory, copying hello.cpp
• Add the source file to meson.build file
• Add #include <js/experimental/JSStencil.h> and some other public headers to the example file
• Rewrite the code around JS::Evaluate with JS::Stencil-related APIs