Support multiline block scalar strings

[MariusStorhaug]

Why

YAML supports multiline block scalars for representing long text values without embedding literal newline escapes, but the parser currently does not recognise them.

What

ConvertFrom-Yaml should parse literal (|) and folded (>) block scalars, such as:

description: |
  This is a multiline string.

Acceptance criteria

• A literal block scalar (|) parses to a string preserving embedded newlines
• A folded block scalar (>) parses to a string with line folding applied per the YAML spec

[MariusStorhaug]

What changed: Reformatted the issue body to the MSXOrg Issue Format — a local Why plus an outcome-level What/acceptance criteria. Removed the Technical Decisions and Implementation Plan sections (and other implementation-tradeoff notes).
Why: Issue bodies should state local purpose and observable outcome without leaking implementation detail into the outcome-level description; durable technical decisions belong in the PR/design, not the tracking issue.
Follow-up: None.