[Accessibility]: `Set Selection Anchor` and `Select from Anchor to Cursor` does not work in a11y terminal buffer

[jooyoungseo]

CC @meganrogge

The following does not work in a11y terminal buffer. Would you please support them?

• Set Selection Anchor (Ctrl+K, Ctrl+B)

• Select from Anchor to Cursor (Ctrl+K, Ctrl+K)

[vscodenpa]

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