feat[lang]!: ban nonreentrant on internal functions

[charles-cooper]

What I did

How I did it

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Commit message

this commit removes the possibility of adding `@nonreentrant` to
internal functions.

first of all, it was a fairly strange feature, since users don't
typically think about marking internal functions as nonreentrant

second of all, it had strange behavior, since a nonreentrant internal
function called by another function (internal or external) would
revert, since the caller had already set the nonreentrancy lock. we
could fix this, but it's a fairly complicated fix.

lastly, with plans to disable reentrancy by default (see GH 3380), the
ideal default behavior of internal functions (not marked with either
`@reentrant` or `@nonreentrant`) is not clear. to avoid this ambiguity,
we simply remove the possibility of nonreentrancy on all internal
functions.

references:
- https://github.com/vyperlang/vyper/issues/3380
- https://github.com/vyperlang/vyper/issues/4509

Description for the changelog

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.70%. Comparing base (2d515d3) to head (44d6cf7).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4573   +/-   ##
=======================================
  Coverage   92.70%   92.70%           
=======================================
  Files         123      123           
  Lines       17526    17527    +1     
  Branches     2971     2971           
=======================================
+ Hits        16248    16249    +1     
  Misses        879      879           
  Partials      399      399           

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[charles-cooper]

@cyberthirst added a sanity check 44d6cf7

[charles-cooper]

so apparently snekmate depends on this -- see https://github.com/pcaversaccio/snekmate/blob/911f13ea72ae465c3a1c1876d1b8aaeaeb92c4ac/src/snekmate/utils/batch_distributor.vy#L43-L45