feat: add bank-account exercise

[metif12]

What

Adds the Bank Account practice exercise (difficulty 5), split out of #307.

• bank-account.v -- stub
• .meta/example.v -- example solution
• run_test.v -- 17 tests, built from the canonical data in problem-specifications
• .meta/config.json, .meta/tests.toml, .docs/
• config.json -- registers the exercise

Adjustments for the test runner's V version

Three adjustments so the exercise builds on the V 0.4.8 that the current test
runner image pins as well as on current V:

• The mutex field is called mutex and is taken with @lock(). lock is a
  reserved keyword in V 0.4.8, so neither the field name nor the method call
  parses there, while @lock() is the name V gives the method on every
  version the track supports.
• balance takes its receiver by value rather than by reference, so the mut
  method behind the mutex can be called at all: V 0.4.8 wants a mut receiver
  variable and rejects mut on a reference receiver. The mutex is a shared
  pointer either way, so the account is still guarded.
• run_test.v waits for its 1000 workers on a buffered channel rather than on
  a sync.WaitGroup. V 0.4.8 rejects calling add/done on a non-mut
  waitgroup, but a mut one shared with spawned functions miscompiles on
  V 0.5.2 and crashes in _InterlockedExchangeAdd64, so no form of the
  waitgroup works on both. A channel does.
• The methods return ! rather than !void, because V 0.4.8 rejects !void.

The stub carries the same adjustments, so a student on V 0.4.8 starts from
something that parses.

One self-contained commit that touches nothing outside the exercise, so it can be merged on its own.

Verification

• bin/verify-exercises-in-docker (the current vlang-test-runner image, V 0.4.8): pass, 17/17 tests.
• V 0.5.2, the version vlang/setup-v installs for bin/test in CI: pass.
• configlet lint passes; v fmt -l and v vet are clean on the new files.

For the merge commit

Not needed: a brand new exercise does not change the outcome of any existing exercise's tests.