Allow the use of underscore (rather than hyphen) to specify languages in settings

[henri-gasc]

Is your feature request related to a problem? Please describe.
I am using ltex-ls on neovim, and as such, it is configured using Lua. However, Lua does not support the use of hyphen in tables, and I need it to (for example) disable a rule in a language.

This code can not work:

disabledRules = {
	-- Work, but does not set en-* languages
	en = {
		"ARROWS",
	},

	-- Syntax error, can not use hyphen
	en-US = {
		"ARROWS",
	},

	-- Syntax error, can not use literal string as key
	"en-US" = {
		"ARROWS",
	},
},

Describe the solution you'd like
The solution would be allowing to replace the hyphen with an underscore in the configuration.

Describe alternatives you've considered
Other solutions includes:

• On the user part:
  • Change the document language to something without hyphen in the name
  • Accept to not be able to disable or enable specific rules
• On ltex-ls part:
  • Internally remove non-letter from language name and set the rules based on it (from en-US/en_US/en-("&'--__-US to enUS)
  • If the value of disabledRules is a string, read from the file whose name we set in it