chore(ci): add action to test abnf syntax and examples in OM2.0 spec

[krajorama]

Action item from OpenMetrics 2.0 WG.

Help from the script:

# This script opens a markdown file containing the OpenMetrics specification,
# extracts the ABNF grammar from it, and checks if the grammar is valid.
# ABNF grammer must be enclosed in
#    ```abnf
#    exposition = metricset HASH SP eof [ LF ]
#    ...
#    ```
# code block, and the top node must be `exposition`.
# It also extracts examples from the OpenMetrics spec file and checks if they
# are valid according to the grammar.
# Exampes must be enclosed in
#    ```openmetrics
#    ... example content ...
#    ```
# code blocks.

[ArthurSens]

LGTM, but I'd wait for @juliusv opinion about whether it's ok to have something like this here. He is the person working in this repository the most

[ArthurSens]

It's probably a good idea to add the scripts here, too

[juliusv]

Copying my comment from Slack here: I'm fine with it, I'd just want it to live completely in its own world so I don't have to think about it 😊 So maybe sharing the TypeScript-based scripts directory is not ideal. Would you mind putting the OM/Python-related things either in its own top-level directory or even into the .github directory somewhere if that doesn't cause issues?