Basic implementation of language server

[aljazerzen]

This PR adds a language server can be started by python -m edb.language_server.main which will then speak LSP over stdin and stdout streams.

It will:

• parse and report any syntactic errors in the opened edgedb files,
• find dbschema/ directory in the workspace,
• parse and compile the schema,
• compile and edgedb files against that schema,
• if any DDL is found, it will apply that DDL during each file compilation.

Example:

# dbschema/default.esdl

module default {
    type Player {
        property username: str;
    };
};

# a.edgeql

create TYPE Clan {
    create property name -> str;
};

alter type Player {
    create required LINK clan: Clan;
};

select default::Player {
    username,
    clan: {
        name
    }
}
filter .username ilike '%pickle%' and .age > 10;

TODO:

• when a schema is changed, recompile it

[aljazerzen]

To check it out, one must:

• checkout this PR into your dev env,
• checkout language-server branch of edgedb-editor-plugin,
• edgedb-editor-plugin in vscode, using the same python venv that your dev env is installed in,
• launch "Run Extension" task,
• now a new vscode window should open and you can navigate to an edgedb project.

[msullivan]

What should we do to test this? Some tests that talk to the language server directly?

[aljazerzen]

Probably yes. Run python -m edb.language_server.main in a subprocess and feed it LSP requests.