Fix tab keybinding from blocking Find menu tab-navigation

[devrelm]

What this PR does / why we need it:

See #7000 (comment)

In short, the extension's tab and tab+shift keybindings are checking for editorFocus instead of editorTextFocus.

editorFocus is defined as:

An editor has focus, either the text or a widget.

emphasis mine

Because editorFocus is true when the user tabs in the editor-level Find menu (or at the very least it's true after tabbing once to focus the "Match Case" button) the user cannot tab past "Match Case" to "Match Whole Word", "Use Regular Expression", or beyond.

This PR updates the keybindings' where clause to use editorTextFocus in place of editorFocus.

Which issue(s) this PR fixes

fixes #7000

Special notes for your reviewer:

[dominicpalmer]

Nice. @J-Fields, please can this be merged?

[J-Fields]

Thanks a lot!