Implement sub-command parsing + add on/word/line/over commands

src/vm/commands/on.ts

@@ -0,0 +1,27 @@
+import {Command, CommandData, ParseContext, StrTerm} from "./command.js";
+import {Executable} from "../parse.js";
+import {LexInput} from "../lexer.js";
+
+export type OnData = {
+    type: 'line'|'word',
+    specific: StrTerm,
+    exec: Executable<CommandData>,
+}
+
+export class On extends Command<OnData> {
+    async attemptParse(context: ParseContext): Promise<OnData> {
+        return {
+            type: context.popKeywordInSet(['line', 'word']).value,
+            specific: context.popTerm(),
+            exec: await context.popExecutable(),
+        }
+    }
+
+    getDisplayName(): string {
+        return 'on'
+    }
+
+    isParseCandidate(token: LexInput): boolean {
+        return this.isKeyword(token, 'on')
+    }
+}