fix(pipe): stop mask pipe from leaking config between calls

[endlacer]

fixes 1669

[NepipenkoIgor]

Thanks for the fix, @endlacer! Verified locally on top of develop: the full lib suite is green (905 tests) and 4 of the 5 new tests fail against the old pipe, so they do cover #1669. A per-pipe NgxMaskService with patterns merged from the provided defaults is the right approach.

Two requests before merge:

1. Please move the tests into mask.pipe.spec.ts as a nested describe('config isolation (#1669)') and drop issue-1669.spec.ts. Pipe regressions belong with the other pipe tests (#1567 and #1492 are already there), and that file has the TestBed setup that createPipe duplicates. The existing issue-NNNN.spec.ts files are a pattern we want to move away from, so please don't add to it.

2. One leak of the same kind remains on a single pipe instance:

   pipe.transform('12', '00');                                              // '12'
   pipe.transform('12', '00', { patterns: { '0': { pattern: /[a-z]/ } } }); // '12', expected ''

   applyMask (ngx-mask.service.ts:311-322) still sees currentValue from the previous call and returns the input unmasked. It pre-dates this PR, so say if you'd rather leave it for a follow-up, but resetting the per-call service state at the top of transform plus a test would close the issue properly.

No need to touch the version or CHANGELOG.md, we'll handle that at release.