Update documentation according to FunC v0.5.0

[tolk-vm]

UPD. We decided not to develop FunC, but to fork it and start developing a new language, named TOLK.
See ton-blockchain/ton#1345

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This MR updates documentation according to FunC v0.5.0.

When ton-blockchain/ton#1026 is merged, this MR will become non-draft

What changes were made to the documentation

• updated grammar in prism-func.js for new keywords and // comments
• all comments were replaced from ;; {- -} to // /* */ in all code snippets
• rewritten a part about function specifiers (impure by default, pure keyword, get methods)
• dropped impure from all code snippets, added pure to stdlib where necessary
• replaced method_id with get in all code snippets
• enumerated all built-in functions in stdlib with builtin keyword, dropped "built-ins" page
• deprecated two pragmas, added remove-unused-functions
• updated operators priority list
• added changelog and migration guide (from v0.4.x to v0.5.0)

Though I've made changes to code snippets in Chinese translation, but I didn't translate added/edited contents. It means, that if we want to keep Mandarin version up to date, all changes from this PR should be mirrored and translated.

Fixes #563

[reveloper]

Though I've made changes to code snippets in Chinese translation, but I didn't translate added/edited contents. It means, that if we want to keep Mandarin version up to date, all changes from this PR should be mirrored and translated.

That's fine! We are working on localization updates separately so that you can avoid these remarks for other languages.