Allow EditorConfigFile to be used in-memory without access or use of physical file system.

[maxkatz6]

Hi @Mpdreamz !

I was going to use EditorConfig in Avalonia XAML compiler, and found couple of blocking problems that I think can be easily avoidable with this PR:

1. Currently EditorConfigParser forces to use editorconfig files from the file system defined in the same folder and up. It is technically a standard behavior, but MSBuild/VisualStudio can create temporal editorconfig files in an intermediate (obj) folder. Our compiler receives an array of file paths as input from the build system, and we should be able to parse them instead of iterating manually.
2. Some APIs accept EditorConfigFile as an input, but it's not possible to actually use them, since constructors were internal. This PRs adds some factory methods (two - one with a file path and the second with a TextReader parameter).
3. This library assumes that developers always have access to the file system, and we can synchronously read from it. It might not be the case in sandboxed environments. Note: it's not really the problem for Avalonia, but I can imagine editorconfig to be usable in online editors in the browser-wasm. This problem is solved by providing Parse(TextReader) overload, which can be used from the in-memory file contents.
4. EditorConfigFile instances can be reused and cached, but this problem was already solved in previous PRs in another way. So it's not really critical.

[maxkatz6]

"directory" parameter wasn't used here. So I removed it just to be sure that EditorConfig without a directory can be used as well. I.e. matching section glob directly to the file name.

[Mpdreamz]

This looks great!

I'm currently on PTO but will look into pulling this in asap early next week!

[Mpdreamz]

LGTM! only had to update the test to be a tad more x-plat friendly.

[Mpdreamz]

This should now be up on NuGet in version 0.15.0 (despite the failed Github Action 😢).