syntax highlight rktd files (langserver should not raise error for `.rktd`)

[jestarray]

I'm told racket data have the extension .rktd, for just data files, as is .json to .js, however racket-langserver nags you if the 1st line isn't a #lang definition.

Missing or invalid #lang lineRacket

We might also want to PR magic racket to support syntax highlighting for .rktd files

[6cdh]

The read-language is here:

racket-langserver/doclib/check-syntax.rkt

Lines 13 to 28 in bafff8b

	(define (get-indenter text)
	(define lang-info
	(with-handlers ([exn:fail:read? (lambda (e) 'missing)]
	[exn:missing-module? (lambda (e) #f)])
	(read-language (open-input-string text) (lambda () 'missing))))
	(cond
	[(procedure? lang-info)
	(lang-info 'drracket:indentation #f)]
	[(eq? lang-info 'missing)
	; check for a #reader directive at start of file, ignoring comments
	; the ^ anchor here matches start-of-string, not start-of-line
	(if (regexp-match #rx"^(;[^\n]*\n)*#reader" text)
	#f ; most likely a drracket file, use default indentation
	; (https://github.com/jeapostrophe/racket-langserver/issues/86)
	'missing)]
	[else #f]))

The diagnostic message is produced here:

racket-langserver/doclib/service/diagnostic.rkt

Lines 45 to 51 in bafff8b

	(when (eq? indenter 'missing)
	(add-diag!
	(Diagnostic #:range (Range #:start (Pos #:line 0 #:char 0)
	#:end (Pos #:line 0 #:char 0))
	#:severity DiagnosticSeverity-Error
	#:source "Racket"
	#:message "Missing or invalid #lang line")))

Add a guard to prevent produce this message for rktd files should fix this.

[dannypsnl]

@jestarray You might like to check #213

[dannypsnl]

Complete at #210