[SPEC] Add a lightweight spec for Portable Mode (#15032)

doc/specs/portable-mode-spec.md

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+---
+author: Dustin L. Howett @DHowett
+created on: 2023-03-22
+last updated: 2023-03-22
+issue id: none
+---
+
+# Windows Terminal "Portable" Mode
+
+## Abstract
+
+Since we are planning on officially supporting unpackaged execution, I propose a special mode where Terminal stores its
+settings in a `settings` folder next to `WindowsTerminal.exe`.
+
+## Inspiration
+
+- [PortableApps](https://portableapps.com)
+- "Embeddable" Python, which relies on the deployment of a specific file to the Python root
+
+## Solution Design
+
+- _If running without package identity,_ `CascadiaSettings` will look for the presence of a file called `.portable` next
+  to `Microsoft.Terminal.Settings.Model.dll`.
+- If that file is present, it will change the settings and state paths to be rooted in a subfolder named `settings` next
+  to `Microsoft.Terminal.Settings.Model.dll`.
+
+Right now, _the only thing_ that makes Terminal not work in a "portable" manner is that it saves settings to
+`%LOCALAPPDATA%`.
+
+## UI/UX Design
+
+_No UI/UX impact is expected._
+
+## Capabilities
+
+- Distributors could ship a self-contained and preconfigured Terminal installation.
+- Users could archive fully-working preconfigured versions of Terminal.
+- Developers (such as those on the team) could easily test multiple versions of Terminal without worrying about global
+  settings pollution.
+
+### Accessibility
+
+_No change is expected._
+
+### Security
+
+_No change is expected._
+
+### Reliability
+
+More code always bears a risk.
+
+### Compatibility
+
+This is a net new feature, and it does not break any existing features. A distributor (or a user) can opt in (or out) by
+adding (or removing) the `.portable` file.
+
+The following features may be impacted.
+
+- **Dynamic Profiles** and **Fragment Extensions**
+  - _No impact expected._ Dynamic profiles will still be generated. If a portable installation is moved to a machine without the dynamic profile source, that profile will disappear.
+- `firstWindowPreference` and `state.json`
+  - _No impact expected._
+  - State is stored next to settings, even for portable installations.
+  - If a dynamic profile was saved in `state` and has been removed, Terminal will proceed as in non-portable mode.
+- Moving an install from Windows 10 to Windows 11 and back
+  - _No impact expected._
+- "Machine-specific" settings, like those about rendering and repainting
+  - _No impact expected._
+  - Terminal does not distinguish settings that are specific to a machine. These settings will move along with the portable install.
+- The shell extension
+  - _No impact expected._
+  - The shell extension will not be registered with Windows.
+  - If we choose to register the shell extension, it is already prepared for running a version of WT from the same directory. Registering the portable shell extension will make it launch portable Terminal.
+
+### Performance, Power, and Efficiency
+
+_No change is expected._
+
+## Potential Issues
+
+- User confusion around where settings are stored.
+
+## Future considerations
+
+- In the future, perhaps `.portable` could itself contain a directory path into which we would store settings.
+- We could consider adding an indicator in the Settings UI.
+- Because we are using the module path of the Settings Model DLL, a future unpackaged version of the shell extension
+  that supports profile loading would read the right settings file (assuming it used the settings model.)
+- If we choose to store the shell extension cache in the registry, we would need to avoid doing so in portable mode.