[Feature Request]: Types for file paths and plain text

### Summary

In documentation/specs/proposed, [security-metadata.md](https://github.com/dotnet/msbuild/blob/1725b247e8737804076c8ff3b09fcee02ecdf51e/documentation/specs/proposed/security-metadata.md) states:

> We envision MSBuild to have a first-class-[citizen] type system for [its] data and tasks. 'Secret' would be one of the data types ...

If a type system is introduced, please include a file path type and a plain text type.

### Background and Motivation

When a `TaskItem` is constructed (or `ItemSpec` is set), the value is passed through `FileUtilities.FixFilePath`:

https://github.com/dotnet/msbuild/blob/1725b247e8737804076c8ff3b09fcee02ecdf51e/src/Utilities/TaskItem.cs#L75-L81

https://github.com/dotnet/msbuild/blob/1725b247e8737804076c8ff3b09fcee02ecdf51e/src/Utilities/TaskItem.cs#L152-L163

https://github.com/dotnet/msbuild/blob/1725b247e8737804076c8ff3b09fcee02ecdf51e/src/Shared/FileUtilities.cs#L543-L546

`FixFilePath` makes no change under Windows (where `Path.DirectorySeparatorChar` will be `'\\'`) but on *NIX OSs (macOS and Linux) a `\` character will be replaced with a `/` character. This makes some sense for file paths but when an Item collection is used for data that is not file paths and that contains `\` characters, the data is corrupted.

This issue shows up in multiple places and ways but one example is using the `ReadLinesFromFile` task to read a JavaScript file that has code using regular expressions. The `\` character is an escape character in REs. The same MSBuild project will work correctly on Windows and corrupt the RE expressions on Linux and macOS.

### Proposed Feature

- Have a plain text type for values that should not be interpreted.
- Have a file path type that supports making file paths portable/convertible.
- Can be defined with a relative path in either a Windows style or a UNIX style and, when its value is accessed, it tries to default to the current platform.
- It should be possible to explicitly get a UNIX style path under Windows and vice versa.
- Converting fully qualified paths may not be supported.

### Alternative Designs

_No response_