Enhancement (config): Add option $mediaLinkFormat to use wikilinks for media

[El1as42]

Closes #213

Builds on #214 (its two commits are the first commits here), since media names must be unique within a notebook before they can be unique across notebooks. I'll rebase once it is merged, so that only the last commit remains.

Changes

• New config option $mediaLinkFormat: 1 uses markdown links with relative paths (default, existing behavior), 2 uses wikilinks with only the file name, e.g. ![[mynotebook-mysection-mypage-image1.png]] for images and [[mynotebook-mysection-mypage-attachment.pdf]] for attachments.
• Since wikilinks contain only the file name, media file names are unique across notebooks with 2:
  • Images and attachments are prefixed with the notebook, Section, and page path, i.e. the .md file's path within the notes destination.
  • # and ^ are replaced, since they have a special meaning in wikilinks.
  • Prefixes longer than 100 characters are shortened to their start (e.g. the notebook) and end (e.g. the page), cut at dashes if possible, with a hash of the full prefix in between, e.g. mynotebook-1a2b3c4d-mypage-image1.png. Attachment names longer than 150 characters are shortened, with a hash of the full name. So names stay unique, and media paths stay well below the path limit of 260 characters of Windows PowerShell 5.1, also after moving the notes to another folder. (Without this, a page nested three subpages deep in a notebook with a long name got image paths of 268 characters, and its images failed to be saved.)
• Images are matched by their prefix, since it may contain any parentheses, e.g. f(g(x)) from nested ones, or a) from a page named a) Exercise. Wikilinks have no alternative text or pandoc link attributes, so these are omitted.
• README.md and config.example.ps1 document the option.

Tests: a New-SectionGroupConversionConfig test for both values: media names are unique across the two fake notebooks, # and ^ are replaced, long subpage paths are shortened with a hash, attachments are prefixed and long attachment names shortened, and images (with alternative text and link attributes, two on one line, nested and unbalanced parentheses) and attachments are converted to wikilinks, or kept as markdown links with 1. Unit tests pass on Windows PowerShell 5.1 and PowerShell 7.0. In this repo's CI (run on my fork), all jobs that can currently run pass: Windows PowerShell 5.1 (windows-2022) and PowerShell 7.1 to 7.4. The other jobs currently fail for any PR, before checkout: the windows-2019 runner image was retired, and the PowerShell 6.0 to 7.0 container images are too old for actions/checkout (GLIBC_2.28' not found). Also tested on real notebook pages on Windows PowerShell 5.1, including a page nested three subpages deep in a notebook with a long name: images are converted to wikilinks, the linked files exist with these names, and the longest media path is 198 characters.

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