Use `+` as marker for path parameters when splitting a specification

Describe the bug

When splitting an existing specification, and individual path files are generated, the parameters in path are enclosed in {} in the names of corresponding generated path files.

The characters { and } need escaping when referred in Linux shell, and this makes harder to manage them.

To Reproduce
Steps to reproduce the behavior:

1. Given any working redocly.yaml file
2. Given any valid openapi-bundle.yaml file having at least one endpoint with path parameters
3. Run openapi split openapi-bundle.yaml --outDir ./split
4. Try to refer a path file relative to an endpoint with a path parameter: it will require something like paths/book_\{bookId\}.yaml

Expected behavior

It would be preferable to have filenames without special characters in their names.

A good candidate seems to be #+, as it looks like it is not needing any escaping in Windows. To preserve retro-compatibility, the marker character could eventually be configurable.

Redocly Version(s)

1.0.0-beta.125

Node.js Version(s)

v18.13.0

Start with the openapi split openapi-bundle.yaml --outDir ./split command and the generated path-file naming behavior described in the issue. Reproduce a split specification containing a path parameter, then trace the CLI entry point that creates those filenames. Done means generated path files use + instead of {} while remaining usable for the documented endpoint references.