Add Fortran Support

[ja72]

Is it as simple as adding the corresponding lines in TextSnapshotTeXCommentBlocks?

public static readonly HashSet<string> SupportedContentTypes = new HashSet<string>() 
{ "CSharp", "F#", "C/C++", "Basic", "Python", "R", "Fortran" };

public static readonly Dictionary<string, string> CommentPrefixPerContentType = new Dictionary<string, string>()
{
    { "CSharp", "//" },
    { "F#", "//" },
    { "C/C++", "//" },
    { "Basic", "'" },
    { "Python", "#" },
    { "R", "#" },
    { "Fortran", "!"},  
};

public static readonly Dictionary<string, string> TeXCommentPrefixPerContentType = new Dictionary<string, string>()
{
    { "CSharp", "//tex:" },
    { "F#", "//tex:" },
    { "C/C++", "//tex:" },
    { "Basic", "'tex:" },
    { "Python", "#tex:" },
    { "R", "#tex:" },
    { "Fortran", "!tex:"},
};

[ja72]

#73 I made the change and tested the add-in locally. Freeform Fortran files now are supported.