fix: support schema content with pathy fileName

[aloisklink]

Prevent an ENOENT error when running jsonschema2md on a JSON Schema that doesn't exist on disk, yet has a pathy fileName.

Using a fileName that is a path is required to get working links to the schema.json file in the output Markdown.

Current behavior

The Public API with non-existent schema path works test (added in this PR) currently fails with the following error.

  1) Testing Public API
       Public API with non-existent schema path works:
     Error: /my-custom-path-to-schema/abstract.schema.json: ENOENT: no such file or directory, open '/my-custom-path-to-schema/abstract.schema.json'
      at Object.openSync (node:fs:601:3)
      at Object.readFileSync (node:fs:469:35)
      at Object.readFileSync (node_modules/jsonfile/index.js:50:22)
      at file:///home/alois/Documents/jsonschema2md/lib/writeSchema.js:42:22
      at Array.map (<anonymous>)
      at file:///home/alois/Documents/jsonschema2md/lib/writeSchema.js:36:24
      at jsonschema2md (file:///home/alois/Documents/jsonschema2md/lib/index.js:153:5)
      at Context.<anonymous> (file:///home/alois/Documents/jsonschema2md/test/api.test.js:103:20)
      at process.processTicksAndRejections (node:internal/process/task_queues:95:5)

Background

I'm trying to simplify some code in https://github.com/mermaid-js/mermaid/blob/b87f1f209843ed794d83bfece5669adf595bdabd/packages/mermaid/scripts/docs.mts#L393-L420 to use the official import {jsonschema2md} from ... API, since it currently uses a bunch of internals.

[codecov]

Codecov Report

Merging #522 (7018c70) into main (119f517) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #522   +/-   ##
=======================================
  Coverage   99.75%   99.75%           
=======================================
  Files          11       11           
  Lines        2039     2039           
=======================================
  Hits         2034     2034           
  Misses          5        5           

Files Changed	Coverage Δ
lib/writeSchema.js	100.00% <100.00%> (ø)

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