Fold template-extension package into core packages

[hwillson]

Migrating over from meteor/meteor#3706.

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(originally opened by @aldeed)

I'd like to make a case for folding the template-extension package API into the core API from blaze/templating packages.

• It's messing with private properties.
• Some of the functions could be implemented in a more ideal way if done alongside the core functions
• Most of the functions are extremely useful, to the point of being needed in pretty much every project I do.
• It's really not much code
• We'd like to add support for template tag attributes, which we can't currently do.
• We can submit PRs with tests if we know MDG will pull them.

[filipenevola]

I'm closing this issue because it's too old.

If you think this issue is still relevant please open a new one.

Why? We need to pay attention to all the open items so we should keep opened only items where people are involved.