Implemented error recovery support in parametric language servers

[PieterOlivier]

This PR leverages the error recovery support in TextDocumentService to also support error recovery for parametric language services.
The Pico demo has been extended with error recovery support.

The first step when supporting error recovery is to change the parser to do error recovery:

set[LanguageService] picoLanguageServer() = {
    parsing(parser(#start[Program], allowRecovery=true, allowAmbiguity=false)),

After that, the individual services need to be able to work even when error trees are present. In case of Pico, only a small change was needed to picoDocumentSymbolService, all other services kept working without changes.

[sungshik]

The analyzer runs on parse trees with errors! 😲 That's beyond my expectations. Really cool 🙂 There are some inconsistencies between analyzer diagnostics and the outline, though, so that requires some additional tweaking. I suspect it might be a bug in version management of diagnostics; see the comment below for more details.

[sungshik]

I fixed the analyzer issue (see comment), but I'm not sure if it addresses all potential issues in the analyzer. Remaining questions:

• This is fine, I think? (Or can id have errors?)

  rascal-language-servers/rascal-lsp/src/main/rascal/demo/lang/pico/LanguageServer.rsc

  Lines 100 to 101 in 5a36693

  	// uses of identifiers
  	rel[loc, str] uses = {<id.src, "<id>"> | /Id id := input};

• Do we need to check hasErrors(var) here, too? Not sure because src is maybe always defined.

  rascal-language-servers/rascal-lsp/src/main/rascal/demo/lang/pico/LanguageServer.rsc

  Lines 103 to 104 in 5a36693

  	// documentation strings for identifier uses
  	rel[loc, str] docs = {<var.src, "*variable* <var>"> | /IdType var := input};

• What about here? (Probably the same as the first question)

  rascal-language-servers/rascal-lsp/src/main/rascal/demo/lang/pico/LanguageServer.rsc

  Lines 119 to 123 in 5a36693

  	// Provide warnings (expensive to compute) only in build mode
  	if (build() := mode) {
  	rel[loc, str] asgn = {<id.src, "<id>"> | /Statement stmt := input, (Statement) `<Id id> := <Expression _>` := stmt};
  	s.messages += {<src, warning("<id> is not assigned", src)> | <id, src> <- defs, id notin asgn<1>};
  	}

[PieterOlivier]

I think in this case everything works becuase lexicals can never be error trees.