[new release] dkml-workflows (1.0.0)

CHANGES:

New Features:
1. Support GitLab CI/CD
2. Support desktop testing on Windows
3. GitHub now uses a composite action rather than a child
workflow, resulting in less artifact copying and
quicker builds.

There are significant breaking changes. It will be far easier
to onboard with [the new version `v1` instructions](https://github.com/diskuv/dkml-workflows/tree/v1#readme)
and then remove your `v0` code, rather than try to do an in-place upgrade:
* Any custom build logic you have in your GitHub workflow should go into
the new `ci/build-test.sh`. Alternatively, if you don't care about ever running troubleshooting
CI on your desktop or GitLab, directly into your new `.github/workflows/build-with-dkml.yml`.

Breaking changes:
- The GitHub child workflow has been replaced by a GitHub composite action
- Input variables have been renamed to allow the same variable names between GitHub Actions and
GitLab CI/CD (the latter does not support dashes in variable names).

| Old Name | New Name |
| ------------------------- | ------------------------- |
| cache-prefix | CACHE_PREFIX |
| ocaml-compiler | OCAML_COMPILER |
| dkml-compiler | DKML_COMPILER |
| conf-dkml-cross-toolchain | CONF_DKML_CROSS_TOOLCHAIN |
| diskuv-opam-repository | DISKUV_OPAM_REPOSITORY |
| ocaml-options | ocaml_options |
| vsstudio-arch | vsstudio_arch |
| vsstudio-hostarch | vsstudio_hostarch |
| vsstudio-dir | vsstudio_dir |
| vsstudio-vcvarsver | vsstudio_vcvarsver |
| vsstudio-winsdkver | vsstudio_winsdkver |
| vsstudio-msvspreference | vsstudio_msvspreference |
| vsstudio-cmakegenerator | vsstudio_cmakegenerator |

- Matrix variables have been renamed to allow the same variable names between GitHub Actions and
GitLab CI/CD (the latter does not support dashes in variable names).

- The shell matrix variable `default_shell` has been renamed `gh_unix_shell`

- The operating system matrix variable has been reorganized to distingush GitHub
from GitLab:

- `os` is now `gh_os` and in use only for GitHub Actions
- `gl_tags` and `gl_image` are the new GitLab CI/CD equivalents. GitLab CI/CD uses tags like
`[shared-windows, windows, windows-1809]` to specify the type of runner machine to use,
and for macOS image you can supply an XCode version like `macos-11-xcode-12`.