Rename telescope extension from todo-comments to todo_comments

[tony-sol]

According to lua syntax, it doesn't support dot-access to table element if key has dash symbol, so this

require('telescope').setup {
	extensions = {
		todo-comments = {},
	},
}

won't work and should be write as

require('telescope').setup {
	extensions = {
		['todo-comments'] = {},
	},
}

Renaming extension to todo_comments would allow to use both variants, like

require('telescope').setup {
	extensions = {
		todo_comments = {},
	},
}
...
require('telescope').setup {
	extensions = {
		['todo_comments'] = {},
	},
}