escapeString in typedoc-plugin-markdown doesn't escape leading quotes

[0x31]

https://github.com/tgreyuk/typedoc-plugin-markdown/blob/e6046b0aa1779fb316fa2448457aa468569c38e5/packages/typedoc-plugin-markdown/src/utils/front-matter.ts#L47

By checking that the quote doesn't already follow an escape, it misses double quotes at the start of the string.

In docusaurus-plugin-typedoc, this can cause issues with the sidebar label:

sidebar_label: ""library-name\""

which causes the error

YAMLException: can not read a block mapping entry; a multiline key may not be an implicit key at line 5, column 16:
    custom_edit_url: null

[tgreyuk]

thanks - fix in 3.8.2