Give "not-ino" dummy file a more explanatory name

[per1234]

Describe the current behavior

The "Problems" view lists problems from files outside the sketch under a dummy file named "not-ino".

https://github.com/arduino/arduino-language-server/blob/0.5.0-rc6/sourcemapper/ino.go#L31

I think this could be confusing to the users, especially since the problems themselves tend to be quite mysterious. I'm wondering if the location of these problems could be made more clear by using another name.

To reproduce

1. Select Tools > Board > Arduino SAMD Boards (32-bits ARM Cortex-M0+) > Arduino MKRZERO from the Arduino IDE menus.
2. Open the command palette (Ctrl+Shift+P).
3. Select "Toggle Problems View"

Note that there is a "not-ino" file that contains some cryptic problems:

Describe the request

Consider other names to use in place of "not-ino". I don't know whether there is some convention for this sort of thing in language servers.

Possibilities:

• non-sketch
• external

Desktop

• OS: Windows 10
• IDE Version: 2.0.0-rc1-snapshot.767b09d
  Date: 2021-12-07T16:40:38.636Z
  CLI Version: 0.20.1 [abb21449]
• Arduino Language Server version: 0.5.0-rc6

Additional context

Part of what made this confusing to me is that "ino" is the file extension for Arduino Programming Language files, but Arduino sketches can contain files of other programming languages that have other file extensions (.h, .cpp, .c, .S). So when I see "non-ino", it sounds like it means files with an extension other than .ino.

[per1234]

The exact contents of the "Problems" view produced by the instructions above have changed since the time I wrote the issue (likely as a result of #92), but the instructions do still demonstrate the "non-ino" issue reported here.