Add .slnLaunch to JSON extensions

[brunt-toast]

.slnLaunch is a json-formatted file used by Visual Studio to create launch profiles for combinations of .NET projects with specific configurations and architectures.

As indicated by Microsoft Learn - Set multiple startup projects and Visual Studio's own behaviour when creating a launch profile via GUI, the canonical format is ".slnLaunch", with the capital "L", however Visual Studio will also recognise the file with a lower-case L. In fact, Visual Studio matches case-insensitively, but .*\.sln[Ll]aunch covers most reasonable cases.

• I am adding a new extension to a language.
  • The new extension is used in hundreds of repositories on GitHub.com
    • Search results for each extension:
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.slnLaunch
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • https://github.com/dotnet/samples/blob/c6eb3268ba912447e820ba78e3d35ee2c5d7e0b0/orleans/ChatRoom/ChatRoom.slnLaunch
    • Sample license(s):
      • CC-BY-4.0
  • I have included a change to the heuristics to distinguish my language from others using the same extension.

(Above: N/A, as far as I am aware there is no other use for .slnLaunch)

[Alhadis]

Linguist identifies file extensions case-insensitively, so there's no need for camelCased variant of .slnLaunch.

Suggested change

	- ".slnlaunch"
	- ".slnLaunch"
	- ".slnlaunch"