[YAML] Multi-line flow scalars' lines starting with vertical bar are highlighted as invalid

[EpicWink]

What happened?

In multi-line flow scalars (ie strings), subsequent lines which start with the vertical bar | are highlighted as invalid (white on red, in my theme), while actually being valid YAML syntax.

.docker-image:build:
  script:
    - imageTag="$(echo "$CI_COMMIT_TAG"
      | grep -Eo '\d+\.\d+\.\d+(-?(a(lpha)?|b(eta)?|rc)\.?\d+)?$'
      || echo "$CI_COMMIT_REF_SLUG")"

[FichteFoll]

FYI, indentation-based syntaxes are really annoying to parse with ST's engine. Things might be easier now with branching, but I wonder if it will help here.

We will probably need a "post-plain scalar" context that checks with a look-ahead whether the line has enough indentation to be considered a continuation of the previously started scalar and that needs to be done for list items and key-value pairs.

By the way, here is what the snippet actually looks like with the YAML syntax (similar but different):