Default initialize a CmdPal mode

[leonMSFT]

Whoops, members are zero initialized in Debug builds but most likely not
in Release builds So, this PR adds a couple of default values to
_currentMode and its associated XAML strings to make cmdpal/ats work
deterministically on first use. I also added a default value to
_anchorKey just to be safe.

Closes #7254

[DHowett]

I'm not in love with how we have to set the text in multiple places. Is it possible to centralize how the text is set to the right text for the mode?

[leonMSFT]

Oh absolutely, I should've been using _switchToMode actually!

[ghost]

Hello @leonMSFT!

Because this pull request has the AutoMerge label, I will be glad to assist with helping to merge this pull request once all check-in policies pass.

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[DHowett]

@msftbot merge this in 5 minutes

[ghost]

Hello @DHowett!

Because you've given me some instructions on how to help merge this pull request, I'll be modifying my merge approach. Here's how I understand your requirements for merging this pull request:

• I won't merge this pull request until after the UTC date Wed, 12 Aug 2020 23:24:58 GMT, which is in 5 minutes

If this doesn't seem right to you, you can tell me to cancel these instructions and use the auto-merge policy that has been configured for this repository. Try telling me "forget everything I just told you".

[ghost]

🎉Windows Terminal Preview v1.3.2382.0 has been released which incorporates this pull request.:tada:

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