Format ranges

A nice feature to have would be the ability to only format specific line (or byte) ranges of a particular file.

Some editors (like vscode) have features that correspond to this
- format modified lines
- format on type
- format on paste

See https://code.visualstudio.com/docs/editor/codebasics#_formatting

Examples of formatters that support this
- [black](https://black.readthedocs.io/en/stable/usage_and_configuration/the_basics.html#line-ranges)
- [prettier](https://prettier.io/docs/en/options.html#range)

No implementation files or tests are named. Start by locating pasfmt's formatter entry point, input handling, and existing formatting tests, then compare the requested line- or byte-range behavior with the linked Black and Prettier references. Done means pasfmt formats only the requested range while preserving the rest of the file, with coverage for the supported range cases.

Written by the indexing model from the issue text.