alias: add missing readonly to internal function

[benmccann]

Rollup Plugin Name: alias

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

If yes, then include "BREAKING CHANGES:" in the first commit message body, followed by a description of what is breaking.

List any relevant issue numbers: #679

Description

entries is specified as readonly here:

plugins/packages/alias/types/index.d.ts

Line 30 in 366380e

entries?: readonly Alias[] | { [find: string]: string };

This change makes the implementation readonly to match the type definition. TypeScript 4 has stricter checks for readonly and will eventually require this change.