make decorators readonly when accessed through context

[ethanniser]

The change is like 2 lines, but seems to have had a ripple effect causing a couple type errors, which I am working to fix

But I would still like your thoughts in the meantime

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This pr:

• changes the type of context to make decorators readonly
• adds a test

Why?

• currently it is a bit confusing what is the difference between state and decorate, to a user it feels like they are the same thing just one ends up in ctx and one in ctx.store
• I would argue they should have a semantic difference, where decorators are immutable (ie a database client, or a config object) and shouldn't be modified, where state is mutable and can be modified
  There is this part from the context page of the docs which I believe shares this sentiment:

store: A global mutable store for Elysia instance

Making decorators readonly makes it more clear when a user should use state vs decorate, and provides additional safety for large, important global values you want ensure aren't modified