refactor: change to not using declare class

types/index.d.ts

@@ -508,6 +508,7 @@ export interface MathJsInstance extends MathJsFactory {
   RelationalNode: RelationalNodeCtor
   SymbolNode: SymbolNodeCtor
 
+  Unit: UnitCtor
   Matrix: MatrixCtor
 
   /**
@@ -4011,11 +4012,7 @@ export interface UnitPrefix {
   scientific: boolean
 }
 
-export declare class Unit {
-  constructor(
-    value: number | BigNumber | Fraction | Complex | boolean,
-    name: string
-  )
+export interface Unit {
   valueOf(): string
   clone(): Unit
   // eslint-disable-next-line @typescript-eslint/no-explicit-any
@@ -4042,19 +4039,16 @@ export declare class Unit {
   value: number
   fixPrefix: boolean
   skipAutomaticSimplification: true
-  /**
-   * Parse a string into a unit. The value of the unit is parsed as number, BigNumber, or Fraction depending on the math.js config setting number.
-   *
-   * Throws an exception if the provided string does not contain a valid unit or cannot be parsed.
-   */
-  static parse: (str: string) => Unit
-  /** Test if the given expression is a unit.
-   *
-   * The unit can have a prefix but cannot have a value.
-   */
-  static isValuelessUnit: (name: string) => boolean
-  /** Instantiate a Unit from a JSON object */
-  static fromJSON: (json: object) => Unit
+}
+
+export interface UnitCtor {
+  new (
+    value: number | BigNumber | Fraction | Complex | boolean,
+    name: string
+  ): Unit
+  parse(str: string): Unit
+  isValuelessUnit(name: string): boolean
+  fromJSON(json: object): Unit
 }
 
 export interface CreateUnitOptions {
@@ -6863,6 +6857,7 @@ export const {
   RelationalNode,
   SymbolNode,
   Matrix,
+  Unit,
 
   uninitialized,
   version,