[JS/TS] Rewrite flexbuffers JS to TS (#6148)

* Partial TS rewrite

* Completed port but bugs remain

* Expose builder function

* Break out and fix stack-value and formatting

ts/flexbuffers/stack-value.ts

@@ -0,0 +1,61 @@
+import { Builder } from './builder'
+import { BitWidth } from './bit-width'
+import { paddingSize, uwidth, fromByteWidth } from './bit-width-util'
+import { ValueType } from './value-type'
+import { isInline, packedType } from './value-type-util'
+
+export class StackValue {
+  constructor(private builder: Builder, public type: ValueType, public width: number, public value: number | boolean | null = null, public offset: number = 0) {
+
+  }
+
+  elementWidth(size: number, index: number): BitWidth {
+    if (isInline(this.type)) return this.width;
+    for (let i = 0; i < 4; i++) {
+      const width = 1 << i;
+      const offsetLoc = size + paddingSize(size, width) + index * width;
+      const offset = offsetLoc - this.offset;
+      const bitWidth = uwidth(offset);
+      if (1 << bitWidth === width) {
+        return bitWidth;
+      }
+    }
+    throw `Element is unknown. Size: ${size} at index: ${index}. This might be a bug. Please create an issue https://github.com/google/flatbuffers/issues/new`;
+  }
+
+  writeToBuffer(byteWidth: number): void {
+    const newOffset = this.builder.computeOffset(byteWidth);
+    if (this.type === ValueType.FLOAT) {
+      if (this.width === BitWidth.WIDTH32) {
+        this.builder.view.setFloat32(this.builder.offset, this.value as number, true);
+      } else {
+        this.builder.view.setFloat64(this.builder.offset, this.value as number, true);
+      }
+    } else if (this.type === ValueType.INT) {
+      const bitWidth = fromByteWidth(byteWidth);
+      this.builder.pushInt(this.value as number, bitWidth);
+    } else if (this.type === ValueType.UINT) {
+      const bitWidth = fromByteWidth(byteWidth);
+      this.builder.pushUInt(this.value as number, bitWidth);
+    } else if (this.type === ValueType.NULL) {
+      this.builder.pushInt(0, this.width);
+    } else if (this.type === ValueType.BOOL) {
+      this.builder.pushInt(this.value ? 1 : 0, this.width);
+    } else {
+      throw `Unexpected type: ${this.type}. This might be a bug. Please create an issue https://github.com/google/flatbuffers/issues/new`
+    }
+    this.offset = newOffset;
+  }
+
+  storedWidth(width = BitWidth.WIDTH8): BitWidth {
+    return isInline(this.type) ? Math.max(width, this.width) : this.width;
+  }
+
+  storedPackedType(width = BitWidth.WIDTH8): ValueType {
+    return packedType(this.type, this.storedWidth(width));
+  }
+
+  isOffset(): boolean {
+    return !isInline(this.type)
+  }
+}