Flex completion style is not available on Emacs < 27

[juergenhoetzel]

• eglot.el (eglot--managed-mode): safety check for available flex style

Fixes this issue on Emacs 26.3:

Debugger entered--Lisp error: (void-function nil)
  nil("" #f(compiled-function (probe pred action) #<bytecode 0x1462fbd>) nil 0)
  #f(compiled-function (style) #<bytecode 0x1463319>)(flex)
  completion--some(#f(compiled-function (style) #<bytecode 0x1463319>) (flex basic))
  completion--nth-completion(1 "" #f(compiled-function (probe pred action) #<bytecode 0x1462fbd>) nil 0 (metadata (display-sort-function . #f(compiled-function (completions) #<bytecode 0x1174711>))))
  completion-try-completion("" #f(compiled-function (probe pred action) #<bytecode 0x1462fbd>) nil 0 (metadata (display-sort-function . #f(compiled-function (completions) #<bytecode 0x1174711>))))
  completion--do-completion(#<marker at 86 in Program.fs\Test1\test\emacs-fsharp-mode\fsharp\github\juergen\home\> 86)
  completion--in-region-1(#<marker at 86 in Program.fs\Test1\test\emacs-fsharp-mode\fsharp\github\juergen\home\> 86)
  #f(compiled-function (start end collection predicate) #<bytecode 0x1cb03f>)(#<marker at 86 in Program.fs\Test1\test\emacs-fsharp-mode\fsharp\github\juergen\home\> 86 #f(compiled-function (probe pred action) #<bytecode 0x1462fbd>) nil)
  apply(#f(compiled-function (start end collection predicate) #<bytecode 0x1cb03f>) (#<marker at 86 in Program.fs\Test1\test\emacs-fsharp-mode\fsharp\github\juergen\home\> 86 #f(compiled-function (probe pred action) #<bytecode 0x1462fbd>) nil))
  #f(compiled-function (funs global args) #<bytecode 0x14632f1>)(nil nil (#<marker at 86 in Program.fs\Test1\test\emacs-fsharp-mode\fsharp\github\juergen\home\> 86 #f(compiled-function (probe pred action) #<bytecode 0x1462fbd>) nil))
  completion--in-region(#<marker at 86 in Program.fs\Test1\test\emacs-fsharp-mode\fsharp\github\juergen\home\> 86 #f(compiled-function (probe pred action) #<bytecode 0x1462fbd>) nil)
  completion-in-region(#<marker at 86 in Program.fs\Test1\test\emacs-fsharp-mode\fsharp\github\juergen\home\> 86 #f(compiled-function (probe pred action) #<bytecode 0x1462fbd>) nil)
  completion-at-point()
  eval((completion-at-point) nil)
  eval-expression((completion-at-point) nil nil 127)
  funcall-interactively(eval-expression (completion-at-point) nil nil 127)
  call-interactively(eval-expression nil nil)
  command-execute(eval-expression)

[joaotavora]

Thanks very much!