Add pattern names in SyntaxInformation and missing usage, syntax information and syntax autocompletion

[maxitg]

Changes

• In the Mathematica options inspector, there is an option called "HighlightMissingArgumentsWithTemplate".
• If you turn it on, in case of missing arguments, instead of displaying a single red arrow, you will get a helpful hint with argument names which can serve as a super fast lightweight documentation (like you would get in, e.g., Xcode after autocompleting a function):

• The names of the arguments are taken from pattern names in SyntaxInformation.

• This PR adds names to all SyntaxInformation except for the built-in RulePlot thus enabling this feature for all SetReplace symbols.

• This also adds missing usage messages, syntax information, and syntax autocompletion.

• Expands argument names in usage messages to be descriptive so that using autocompleted templates are more informative.

Comments

• Null is due to OptionsPattern[]. It can be changed to, say, opts if that pattern is named as opts : OptionsPattern[]. However, if one does that, some optional arguments will always be displayed in the template, which is, of course, unacceptable. I think we'll have to wait until this weed is fixed upstream:

• Autocompletion is generally disabled for option names. The problem is that I'm not aware of a way to enable autocompletion for multiple arguments at a time. In order to solve this, we need to use symbols instead of strings for options (#604), and we need to figure out how to autocomplete the values for enum-like options (#605).

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This change is 

[maxitg]

@daneelsan, is there any reason not to merge this now while we try to figure out how to do autocompletion for options?

[daneelsan]

@maxitg I missed the last changes you made. I'll review it now.