A script like the following was used. Note that "pre-commit run -a" was
also executed to reformat the json files afterward.

```
import json
import sys
from pathlib import Path

extensions_index_dir = Path("/home/jcfr/Projects/ExtensionsIndex")
updated_extensions_index_dir = extensions_index_dir

def parse_s4ext(ext_file_path):
    """Parse a Slicer extension description file.
    :param ext_file_path: Path to a Slicer extension description file.
    """
    ext_metadata = {}
    with open(ext_file_path) as ext_file:
        for line in ext_file:
            if not line.strip() or line.startswith("#"):
                continue
            fields = [field.strip() for field in line.split(' ', 1)]
            assert(len(fields) <= 2)
            ext_metadata[fields[0]] = fields[1] if len(fields) == 2 else None
    return ext_metadata

s4ext_filepaths = list(extensions_index_dir.glob("*.s4ext"))

print(f"Found {len(s4ext_filepaths)} extension files (.s4ext)")

for index, filepath in enumerate(s4ext_filepaths):

    metadata = parse_s4ext(filepath)
    updated_metadata = {
        "scm_url": metadata["scmurl"],
        "scm_revision": metadata["scmrevision"],
        "build_dependencies": [] if metadata.get("depends", "NA") == "NA" else metadata["depends"].split(" "),
        "category": metadata["category"],
        "build_subdirectory": metadata["build_subdirectory"],
    }

    with open(updated_extensions_index_dir / f"{filepath.stem}.json", 'w') as fileContents:
        fileContents.write(json.dumps(updated_metadata, sort_keys=True, indent=2))
        fileContents.write("\n")

print(f"Generated {index + 1} extension files (.json)")

from pprint import pprint as pp

print(f"\nMetadata of extension #{index + 1} ({filepath.stem}):\n")
pp(updated_metadata)
```

Update GitHub and CircleCI settings to check for json files.

Update `check_description_files` CLI to parse json files:
* Remove `parse_s4ext` and introduce `parse_json` function along with
  `ExtensionParseError` exception.
* Update list of supported URL schemes. Only `https` and `git` are supported
* Remove obsolete `check_scm_notlocal` check

A script like the following was used. Note that "pre-commit run -a" was
also executed to reformat the json files afterward.

```
import json
import sys
from pathlib import Path

extensions_index_dir = Path("/home/jcfr/Projects/ExtensionsIndex")

json_filepaths = list(extensions_index_dir.glob("*.json"))
print(f"Found {len(json_filepaths)} extension files (.json)")

for index, filepath in enumerate(json_filepaths):
    with open(filepath) as fileContents:
        metadata = json.load(fileContents)
        metadata["$schema"] = "https://raw.githubusercontent.com/Slicer/Slicer/main/Schemas/slicer-extension-catalog-entry-schema-v1.0.0.json#"

    with open(extensions_index_dir / filepath, 'w') as fileContents:
        fileContents.write(json.dumps(metadata, sort_keys=True, indent=2))
        fileContents.write("\n")
```