Add PromptSymbol to Theme

[mdgrs-mei]

Warning

This is small but a breaking change

This PR adds PromptSymbol property to Theme and moves the default prompt "> " from $option.Prompt to $option.Theme.PromptSymbol.

Previously, even if an user customizes the prompt character, the prompt is reset by nested menus like the File Manager.

$option = Get-PSRunDefaultSelectorOption
$option.Prompt = '👉 '
Set-PSRunDefaultSelectorOption $option

By separating the PromptSymbol setting, it can keep the symbol in the nested menus.

This kind of advanced setting becomes possible too.

$option.Theme.PromptForegroundColor = [PowerShellRun.FontColor]::White
$option.Theme.PromptBackgroundColor = [PowerShellRun.FontColor]::Magenta
$option.Theme.PromptSymbol = "$($PSStyle.Foreground.Magenta)$($PSStyle.Background.Black) $($PSStyle.Reset)"
$option.Prompt = 'Filter'