feat: move internal modifier out of experimental

[Copilot]

The internal access modifier has been stable with no issues, so it no longer needs the experimental warning. This removes the experimental-feature diagnostic emitted on every use of internal and cleans up all associated suppressions.

Changes

• modifiers.ts — Remove the block that emitted experimental-feature (messageId: "internal") whenever the internal modifier was used
• messages.ts — Remove the internal messageId from the experimental-feature diagnostic definition
• prototypes.tsp — Remove #suppress "experimental-feature" from internal extern dec (decorator declarations were never independently experimental; the suppress was only needed for internal)
• visibility.tsp, private.decorators.tsp — Retain existing #suppress "experimental-feature" on internal extern fn declarations — function declarations (fn) remain experimental
• internal.test.ts — Update tests to reflect internal no longer emits any diagnostic; previously-allowed usages that only expected experimental-feature now expect clean compilation
• access-modifiers.md — Remove the "Suppressing the experimental warning" section

Before / After

// Before: emits experimental-feature warning, requires suppression
#suppress "experimental-feature" "suppress internal warning"
internal model MyInternalModel {}

// After: no warning, no suppression needed
internal model MyInternalModel {}

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@typespec/compiler@10855

commit: 9bc7f30

[azure-sdk-automation]

You can try these changes here

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