This adds a new device info query for the maximum workgroup/block size
for each dimension.

- Update the main README to reflect the current project status
- Rework the main API generation documentation. General fixes/tidying,
but also spell out explicitly how to make API changes at the top of the
document since this is what most people will care about.

---------

Co-authored-by: Martin Grant <martingrant@outlook.com>

Fix a couple of unhandled edge cases in offload-tblgen that were found
by static analysis
* `LineStart` may wrap around to 0 when processing multi-line strings.
The value is not actually being used in that case, but still better to
explicitly handle it
* Possible unchecked nullptr when processing parameter flags

All entry points (except olInit) now check that offload has been
initialized. If not, a new `OL_ERRC_UNINITIALIZED` error is returned.

Rather than having a number of static local variables, we now use
a single `OffloadContext` struct to store global state. This is
initialised by `olInit`, but is never deleted (de-initialization of
Offload isn't yet implemented).

The error reporting mechanism has not been moved to the struct, since
that's going to cause issues with teardown (error messages must outlive
liboffload).

This removes the `ol_impl_result_t` helper class, replacing it with
`llvm::Error`. In addition, some internal functions that returned
`ol_errc_t` now return `llvm::Error` (with a fancy message).

A new ErrorCode enumeration is present in PluginInterface which can
be used when returning an llvm::Error from offload and PluginInterface
functions.

This enum must be kept up to sync with liboffload's ol_errc_t enum, so
both are automatically generated from liboffload's enum definition.

Some error codes have also been shuffled around to allow for future
work. Note that this patch only adds the machinery; actual error codes
will be added in a future patch.

~~Depends on #137339 , please ignore first commit of this MR.~~ This has
been merged.

Implement the complete initial version of the Offload API, to the extent
that is usable for simple offloading programs. Tested with a basic SYCL
program.

As far as possible, these are simple wrappers over existing
functionality in the plugins.

* Allocating and freeing memory (host, device, shared).
* Creating a program 
* Creating a queue (wrapper over asynchronous stream resource)
* Enqueuing memcpy operations
* Enqueuing kernel executions
* Waiting on (optional) output events from the enqueue operations
* Waiting on a queue to finish

Objects created with the API have reference counting semantics to handle
their lifetime. They are created with an initial reference count of 1,
which can be incremented and decremented with retain and release
functions. They are freed when their reference count reaches 0. Platform
and device objects are not reference counted, as they are expected to
persist as long as the library is in use, and it's not meaningful for
users to create or destroy them.

Tests have been added to `offload.unittests`, including device code for
testing program and kernel related functionality.

The API should still be considered unstable and it's very likely we will
need to change the existing entry points.

https://github.com/codespell-project/codespell

% `codespell
--ignore-words-list=archtype,hsa,identty,inout,iself,nd,te,ths,vertexes
--write-changes`

Summary:
The previous offloading entry type did not fit the current use-cases
very well. This widens it and adds a version to prevent further
annoyances. It also includes the kind to better sort who's using it.

The first 64-bytes are reserved as zero so the OpenMP runtime can detect
the old format for binary compatibilitry.