Fix for filename -> js identifier conversion has broken conversion

[RoystonS]

Pull #157 has broken the conversion of filenames that contain numbers in the middle. Previously the filename 'BodyText2' would have produced a component named 'BodyText2', but now the code incorrectly produces 'BodyText' instead because it's stripping numbers from the entire string.

The comment in the code states that identifiers must start with a letter, but the code beneath it doesn't only remove numbers at the start: they're stripped wherever they appear.

2dfb833#diff-6518fffc4e430fcc63cd30eb07131a95R848

Adding a new unit test shows the problem, as it fails:

it('does not remove non-leading non-letters', () => {
  const result = getDefaultExportForFile({ fileName: 'Body2Text' } as ts.SourceFile);
  assert.equal(result, 'Body2Text');
});

I'll raise a PR. (PR #168)

[pvasek]

It has been released in v1.12.3. Sorry, somehow overlooked your PR.