excerpt_separator option is missing in options schema for gatsby-transformer-remark plugin

[smashercosmo]

Description

According to the docs you should be able to provide 'excerpt_separator' options to gatsby-transformer-remark plugin
https://www.gatsbyjs.com/plugins/gatsby-transformer-remark/?=gatsby-tra#excerpts-for-html-embedded-in-markdown-files

Steps to reproduce

Provide 'excerpt_separator' option to gatsby-transformer-remark plugin and try to run the project

Expected result

No error should be raised

Actual result

error Invalid plugin options for "gatsby-transformer-remark":

- "excerpt_separator" is not allowed

Environment

  System:
    OS: macOS 10.15.7
    CPU: (16) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
    Shell: 5.7.1 - /bin/zsh
  Binaries:
    Node: 14.15.0 - /usr/local/bin/node
    npm: 6.14.8 - /usr/local/bin/npm
  Languages:
    Python: 2.7.16 - /usr/bin/python
  Browsers:
    Chrome: 86.0.4240.111
    Safari: 14.0
  npmPackages:
    gatsby: 2.25.0 => 2.25.0 
    gatsby-image: 2.4.21 => 2.4.21 
    gatsby-plugin-netlify: 2.4.0 => 2.4.0 
    gatsby-plugin-netlify-cms: 4.3.17 => 4.3.17 
    gatsby-plugin-netlify-cms-paths: 1.3.0 => 1.3.0 
    gatsby-plugin-postcss: 2.3.12 => 2.3.12 
    gatsby-plugin-preact: 4.0.16 => 4.0.16 
    gatsby-plugin-react-helmet: 3.3.14 => 3.3.14 
    gatsby-plugin-sharp: 2.7.0 => 2.7.0 
    gatsby-plugin-sitemap: 2.5.0 => 2.5.0 
    gatsby-plugin-svgr: 2.0.2 => 2.0.2 
    gatsby-plugin-typescript: 2.5.0 => 2.5.0 
    gatsby-plugin-webpack-bundle-analyzer: 1.0.5 => 1.0.5 
    gatsby-remark-images: 3.4.0 => 3.4.0 
    gatsby-source-filesystem: 2.4.0 => 2.4.0 
    gatsby-transformer-remark: 2.9.0 => 2.9.0 
    gatsby-transformer-sharp: 2.5.21 => 2.5.21 
    gatsby-transformer-yaml: 2.4.15 => 2.4.15 

[mxstbr]

Thanks for reporting this! PR with a first fix is open at #27785, follow that for updates 👍