code actions for dsls

[jurgenvinju]

This PR connects the codeActions LSP server service for the parametrized LSP server (so for DLSs).

• A CodeActionContributor is of type list[Command] (list[Tree] focus) and registered with data LanguageService = codeActionContributor(CodeActionContributor actions). The "focus" tells us which parse trees are active on the given line and column of the current cursor, ordered with the most specific (smallest) trees first and ending always with the start tree.

Pico example, where we are looking for a cursor position over an IdType somewhere in a start[Program], and we register the command removeDecl which can be interpreter later:

list[Command] picoActions([*_, IdType x, *_, start[Program] program]) 
    = [removeDecl(program, x, title="remove <x>")];

• Normal Commands are used just like the lenses. They need to be evaluated by the commandExecutor contribution.
• Additionally, every Message can now have a keyword parameter: data Message(list[Command] fixes=[]). Every message that is registred with the LSP client (e.g. via a Summary), will lead to a contributed code action when the cursor is at the at location of the info, warning or error message.
• demo::lang::pico::LanguageServer demonstrates these two different ways of contributing actions (see above). Below the Message alternative:

s.messages += {<src, error("<id> is not defined", src, 
                                               fixes=[changeToFix(src, existing<0>, title="Change to <existing<0>>") | existing <- defs])
                             > 
                            | <src, id> <- uses, id notin defs<0>};

The error constructor now receives an additional fixes keyword field with a list of options on how to fix the broken variable reference. Each generated command fixes the bug in its own way (with a different variable name to use).

[jurgenvinju]

Will post some screenshots later. This is still a draft.

[sonarcloud[bot]]

Quality Gate failed

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[jurgenvinju]

lightbulb appears next to error message:

CMD+dot activates menu:

Selecting the right option fixes the issue:

Code for the quickfix:

// attaches multiple quickfix command options to every error (one for every proposed new name):
 s.messages += {<src, error("<id> is not defined", src, fixes=[changeToFix(src, existing<0>, title="Change to <existing<0>>") | existing <- defs])> 
                  | <src, id> <- uses, id notin defs<0>};

// executes the command:
value picoCommands(changeToFix(loc src, str newName)) {
    applyDocumentsEdits([changed(src.top, [replace(src, newName)])]);
    return ("result": true);
}

[jurgenvinju]

Looking for suggestions on how to improve the asynchronous code and suggestions how to test the UI part of this. It's most LSP integration code. The algorithm for generating focus lists could be tested as a unit.