Schema diagnostics in language server

[aljazerzen]

Ref #7411

• adds edb ls sub command, which can be invoked from the vscode extension directly, which makes development much easier,
• treats both .esdl and .gel files as schema files,
• keeps track of modified dbschema/ files (even those that have not been saved to fs yet),
• reports compilation errors from schema (we have previously done that for .edgeql files only)

[scotttrinh]

Latest changes look good to me based on the requests from Sully, so if we feel good about the rest of it,