Add support for YAML HEREDOC highlighting

[sambostock]

Motivation

This adds support for YAML syntax highlighting in HEREDOCs.

Implementation

TDD: The tests added in #842 were updated, and the config file was updated to make them pass.

Automated Tests

TDD

Manual Tests

See the last column in this table.

GitHub		VSCode
Raw YAML	HEREDOC	Raw YAML	HEREDOC Before	HEREDOC After
<<~YAML foo: - [123, abc] - [true, "false"] - { xyz: null} YAML	foo: - [123, abc] - [true, "false"] - { xyz: null}

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Before Merging

• Rebase after Add tests for HEREDOC patterns #842 is merged

[vinistock]

Awesome!