[metif12]

What this adds

A code action that generates a test skeleton.

V has no test framework, so the file layout is the whole contract: foo.v is
tested by foo_test.v beside it, and a test file holds only test_ functions.
Remembering that convention is a small tax on every new file.

On a .v file with public functions, V: Generate Test File creates the
_test.v file beside it with one test_ function per public function, each with
a placeholder assertion to replace.

Design

• Offered on .v files only, and only when the source has public functions. A
  module with nothing public has nothing to test, and an empty test file is worse
  than no test file.
• Refuses to overwrite an existing test file, because that would discard work the
  user already wrote.
• The generation lives in a pure module so it can be tested without an editor.

Verification

npm run lint, npm run lint:md, npm run test:grammar, npm run build and
npm run package all pass.

npm test reports 26 tests, 23 pass, 3 fail. The three failures are the
pre-existing Windows path-separator tests on master, fixed in
#561 and not touched here.

Proof the new tests catch something. I broke the public-function regex in
testSkeleton.ts and watched generates one test function per public function
go red, then reverted it.

Known limitations

• Not exercised in a real VS Code window. The code action provider and the file
  creation are not covered by the extension-host suite; the generation logic is.
• The placeholder assertion is assert true, which always passes. The user
  replaces it with a real assertion. A generated test that passes is not
  evidence that it tests anything.
• Only top-level pub fn declarations are found. A public function inside a
  struct or a pub fn with a receiver is not matched, because the regex is
  anchored to column zero and V has no nested function declarations.

Relation to other PRs

Independent of #556, #557, #558, #559, #560, #561, #562, #563 and #564. It adds
new files and touches extension.ts and package.json only where the other PRs
do not.