[Accessibility]: add two-dimensional navigation the CodeMirror blob view

[taras-yemets]

Audit type

Screen reader navigation

User journey audit issue

No response

Problem description

Follow-up on https://github.com/sourcegraph/sourcegraph/pull/44148.
Currently in the CodeMirror blob view a screen reader user can navigate blame and code content line by line. There's no possibility to go through only blob column or only code column line by line. For more details please check out this comment.

Expected behavior

A screen reader user should be able to navigate the code content in two dimensions: columns (blame or code) and rows (blame and code combined). For more details please check out this comment.

Additional details

No response

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Owner

This issue will be fixed by my team, I have assigned a relevant member to it, or I will do so in the near future.

/cc @felixfbecker