Add GitHub Action to label any PR that affects the PURL Ecma standard

[mjherzog]

We should add a GitHub Action to automatically label any PR that changes a file included in the PURL Ecma standard (1st edition).
At this point that means:

• Any file in /docs/standard/
• Any file in 'schemas/' except for `purl-test.schema-0.1.json' because the test schema is not included in the current 1st edition standard.
• purl-type-definition.schema-1.0.json
• purl-types-index.schema-1.0.json
• purl-type-definition.schema-1.1.json
• purl-types-index.schema-1.1.json

For the v1.0 schema files we want to block any change because these are now frozen and should not be changed.
For the docs and the v1.1 schema files we should apply the current "Ecma standard label" automatically (or some other label)