(core) move home server into core

Summary: This moves enough server material into core to run a home server.  The data engine is not yet incorporated (though in manual testing it works when ported).

Test Plan: existing tests pass

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2552

app/common/plugin.ts

@@ -0,0 +1,77 @@
+/**
+ * Plugin's utilities common to server and client.
+ */
+import {BarePlugin, Implementation} from 'app/plugin/PluginManifest';
+
+export type LocalPluginKind = "installed"|"builtIn";
+
+export interface ImplDescription {
+  localPluginId: string;
+  implementation: Implementation;
+}
+
+export interface FileParser {
+  fileExtensions: string[];
+  parseOptions?: ImplDescription;
+  fileParser: ImplDescription;
+}
+
+// Deprecated, use FileParser or ImportSource instead.
+export interface FileImporter {
+  id: string;
+  fileExtensions?: string[];
+  script?: string;
+  scriptFullPath?: string;
+  filePicker?: string;
+  filePickerFullPath?: string;
+}
+
+/**
+ * Manifest parsing error.
+ */
+export interface ManifestParsingError {
+  yamlError?: any;
+  jsonError?: any;
+  cannotReadError?: any;
+  missingEntryErrors?: string;
+}
+
+/**
+ * Whether the importer provides a file picker.
+ */
+export function isPicker(importer: FileImporter): boolean {
+  return importer.filePicker !== undefined;
+}
+
+/**
+ * A Plugin that was found in the system, either installed or builtin.
+ */
+export interface LocalPlugin {
+  /**
+   * the plugin's manifest
+   */
+  manifest: BarePlugin;
+  /**
+   * The path to the plugin's folder.
+   */
+  path: string;
+  /**
+   * A name to uniquely identify a LocalPlugin.
+   */
+  readonly id: string;
+}
+
+export interface DirectoryScanEntry {
+  manifest?: BarePlugin;
+  /**
+   * User-friendly error messages.
+   */
+  errors?: any[];
+  path: string;
+  id: string;
+}
+
+/**
+ * The contributions type.
+ */
+export type Contribution = "importSource" | "fileParser";