accDescription fields cause parser errors in Sequence Diagrams

[pje]

Describe the bug

Sequence diagrams containing accDescription <foo> or accTitle <foo> cause a parser error.

Several of the sequence diagrams in the demo/ directory have accDescriptions and are currently causing parser errors. For example, demos/sequence.html looks like this currently:

To Reproduce

sequenceDiagram
  accDescription Test a description

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☝️ the sequence diagram above renders an error at the time this issue was opened. The error looks like this in GitHub's UI.

Expected behavior
accDescription fields should be supported in sequence diagrams. No parser errors should happen when rendering them.

Code Sample

https://mermaid.live/edit#pako:eNpFzrEKwzAMBNBfMZrzBZ7TL2hHLcK6tIZYTm15KCH_3gwp3Y7jcdxOqSooUsd7wBLmLM8mhY1dUprRU8ub52rhge5Bgv4rmqigFcl6DuxsITD5CwVM8YyKRcbqTGzHScem4rhp9tooLrJ2TCTD6_1jiaK3gR-6Tlzq-AIiujrd

Desktop (please complete the following information):

• OS: macOS
• Browser Chrome
• Version 105.0.5195.125 (Official Build) (x86_64)

Additional context

Accessible Descriptions (accDescription) fields were added in #3008.

[pje]

Looking at this further I notice that the accTitle: <foo> syntax parses correctly (note the :) (although it does fail to output a <desc> tag):

sequenceDiagram
  accTitle: foo

sequenceDiagram
  title: foo
  accTitle: foo

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So I guess this issue becomes: which is the correct syntax? accTitle: foo or accTitle foo?

From the grammar file I'd assume that accTitle foo without the colon is preferred, to match the title syntax.

mermaid/src/diagrams/sequence/parser/sequenceDiagram.jison

Lines 65 to 66 in c93c7c7

	"title"\s[^#\n;]+ return 'title';
	"title:"\s[^#\n;]+ return 'legacy_title';

[pje]

Created a branch with tests for both syntaxes here: https://github.com/pje/mermaid/tree/bug/3506_sequence_accDescription

[weedySeaDragon]

@pje I had a similar question when looking at the demo files (what should the correct syntax be?).
The documentation is clear: there should be a colon after the the keywords: accTitle: and accDescr:

Some of the demo files definitely need to be fixed.