Give the web runtime support for resources

[Michael-F-Bryan]

In #273 we added resources to rune build and the Rust runtime, but because the web runtime can't be used outside the browser (#270) we didn't detect that the web runtime doesn't support resources.

We need to add the following host functions to the web runtime.

extern "C" {
    ...

    /// Open a named resource, returning a unique ID that can be used to .
    ///
    /// Invalid parameters will return a negative value.
    pub fn rune_resource_open(name: *const u8, name_len: u32) -> i32;

    /// Read data from a resource into the provided buffer.
    ///
    /// Invalid parameters will return a negative value.
    pub fn rune_resource_read(
        resource_id: u32,
        buffer: *mut u8,
        buffer_len: u32,
    ) -> i32;

    /// Close a resource.
    ///
    /// Invalid parameters will be ignored.
    pub fn rune_resource_close(resource_id: u32);
}

(source)

This implements a file-like API, where you'll open a resource and get a handle back (or a negative number indicating something went wrong), keep reading until EOF, then close the resource. I imagine the API changes will be something like this:

export interface Imports {
    ...
    openResource(name: string): Resource;
}

export interface Resource {
  // Read data from this Resource into the destination array, returning the 
  // number of bytes written.
  read(dest: ArrayBuffer): number;
  // Close this Resource, releasing any system resources it may be using.
  // Implementations can assume this method will be called at most once.
  close(): void;
}