Enhance PoshGitTextSpan support for custom VT seqs

[rkeithhill]

@dahlbyk See what you think about this. In the process of explaining for #612 how to use the CustomAnsi property on PoshGitTextSpan to customize built-in prompt, I realized my previous impl of this idea was too limiting. For instance before the Path is display the user wanted this displayed in the prompt:

The ideal property to use for this info is the $GitPromptSettings.DefaultPromptPrefix but there are four different bits of info displayed with different colors. That currently does NOT work.

My original idea with CustomAnsi was that folks could specify a custom sequence that would apply to the text. That would allow the user to set other attributes like say underline or inverse as well as fg/bg colors. But that sequence would apply to all of the text so you could have only a single span of text with those attributes.

Now I'm thinking perhaps we should just ditch the CustomAnsi property and allow folks to put as much custom seqs in the Text field as their heart desires. For instance, with this change, I can achieve the above desired display with this one setting:

function Test-Administrator { $true }

$GitPromptSettings.DefaultPromptPrefix.Text =
    "`n`$(Get-PromptConnectionInfo -Format `"[{1}@{0}]: `")" +
    "`$(if (global:Test-Administrator) {`"$([char]27)[37m^`"})" +
    "$([char]27)[33m`${env:USERNAME}@$([char]27)[95m`${env:COMPUTERNAME}$([char]27)[90m:"

Thoughts?

[dahlbyk]

Now I'm thinking perhaps we should just ditch the CustomAnsi property and allow folks to put as much custom seqs in the Text field as their heart desires.

👍

[rkeithhill]

I'll fix the tests tonight. First, I wanted to see if you agreed in principle to this change.

[rkeithhill]

Oh yeah baby, fixed the AppVeyor tests. I submit a separate PR for that.

[rkeithhill]

@dahlbyk This is ready for a final review/merge.

[rkeithhill]

One thing I wonder about the current impl is that if the user embeds VT seqs in the .Text property without a terminating seq, we write that string "as-is" when AnsiConsole is set to $false. That can mess up the screen colors. In Write-Prompt we could A) strip out seqs, B) add a terminating seq if we detect seqs or C) leave it as it is. If the user directly evals .Text then there's nothing we can do.