test_readme.py: Validate backticked and single-quoted terms in Markdown

[jpmckinney]

We could check that all the following exist in the schema and codelists:

1. JSON paths, field names, definition names ("building block"), codelist names in backticks:

• extension readmes
• documentation websites
• schema description fields

2. codes in single quotes

• extension readmes
• documentation websites
• schema description fields

3. lowerCamelCase and UpperCamelCase words

• extension readmes
• documentation websites
• schema description fields

[jpmckinney]

Added test to check that backticked text in extension readmes is either: JSON paths, field names, definition names ("building block"), codelist names, codelist columns, JSON Schema properties, JSON null, JSON booleans, or merge properties.

[jpmckinney]

We can consider extending the tests to translations.

We could also go further and ensure that backticked content is identical between the source and translations.