First step towards #8415:
* Introduce `PaletteItem` and derive from it to provide native support
for tabs and command lines (`ActionPaletteItem` / `TabPaletteItem`,
`CommandLinePaltteItem`)
* Remove business logic behind PaletteItem from palette (aka dispatch
commands and preview tabs externally)
* Created a ViewModel class in the Command Palette called
FilteredCommand, aggregating the Command, the filter and the
highlighted presentation of the command name
* This ListView of the filtered commands is bound to the vector of
FilteredCommands
* Introduced HighlightedTextControl user control with HighlightedText
view model
* Added this control to the ListView Item's grid
* Bound the FilteredCommand's highlighted command name to the user
control
## Validation Steps Performed
* UT for matching algorithm
* Only manual tests
* Searching in CommandLine, SwitchTab and Nested Command modes
* Checking for bot matching an non matching filters
* Dogfooding
Closes#6646
