Add Coconut language

[rtbs-dev]

After this jupyter-related issue, Coconut has up-to-date and easy-to-use interactions with the jupyter kernel.

I've set up the test notebook to mimic the home-page of the project, though examples that require extra definitions have been commented out.

• Tests were successful: 2441 passed, 2 skipped in 40.37s
• pre-commit hooks successful:

[INFO] Initializing environment for https://github.com/pre-commit/pre-commit-hooks.
[INFO] Initializing environment for https://gitlab.com/pycqa/flake8.
[INFO] Initializing environment for https://github.com/psf/black.
[INFO] Installing environment for https://github.com/pre-commit/pre-commit-hooks.
[INFO] Once installed this environment will be reused.
[INFO] This may take a few minutes...
[INFO] Installing environment for https://gitlab.com/pycqa/flake8.
[INFO] Once installed this environment will be reused.
[INFO] This may take a few minutes...
[INFO] Installing environment for https://github.com/psf/black.
[INFO] Once installed this environment will be reused.
[INFO] This may take a few minutes...
Check JSON...........................................(no files to check)Skipped
Check Yaml...........................................(no files to check)Skipped
Fix End of Files.........................................................Passed
Trim Trailing Whitespace.................................................Passed
flake8...................................................................Passed
black....................................................................Passed
[master 578f8e1] add coconut language
 8 files changed, 1006 insertions(+)
 create mode 100644 tests/notebooks/ipynb_coconut/coconut_homepage_demo.ipynb
 create mode 100644 tests/notebooks/mirror/ipynb_to_Rmd/coconut_homepage_demo.Rmd
 create mode 100644 tests/notebooks/mirror/ipynb_to_hydrogen/coconut_homepage_demo.coco
 create mode 100644 tests/notebooks/mirror/ipynb_to_md/coconut_homepage_demo.md
 create mode 100644 tests/notebooks/mirror/ipynb_to_myst/coconut_homepage_demo.md
 create mode 100644 tests/notebooks/mirror/ipynb_to_percent/coconut_homepage_demo.coco
 create mode 100644 tests/notebooks/mirror/ipynb_to_script/coconut_homepage_demo.coco

[mwouts]

Awesome... and just in time for release 1.5.0 (coming soon). Thank you @tbsexton !