Accept readonly arrays in schema options

[dev054]

Checks before posting an issue

• I have read all the library's documentation (README, CHANGELOG, MIGRATION and /docs)
• I understand I am writing to a human being, doing this open source project on his free time
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Configuration

• @ngx-pwa/local-storage version: 10.x
• Angular version (ng version): 10.x

Description of the issue

Currently the options enum, items and required only support mutable arrays.

How to reproduce the issue

Example of enum:

const themes = ['dark', 'light'] as const;
this.storageMap.get<Theme>('theme', { enum: themes }) 
// The type 'readonly ["dark", "light"]' is 'readonly' and cannot be assigned to the mutable type 'string[]'.

[cyrilletuzi]

Note for myself: check upon TypeScript 4.1 due to microsoft/TypeScript#39258.

[cyrilletuzi]

Released in v11.0.0-1.