`--strip-generated-meta` should narrow all stripping to `platforms/`

> issued from: https://github.com/AdguardTeam/FiltersRegistry/pull/1226#discussion_r3782720598

In `scripts/build/strip-generated-meta.ts`, files are currently matched by filename only, without any folder restrictions. `walkAndStrip` strips any `.txt` file inside any directory named `filters`, and any `filters.json`/`filters.js` (working on #1208) by name, regardless of where in the tree they sit.

We should only strip `.txt` files and `filters.json`/`filters.js` files when they have a `platforms/` ancestor in their path. Files under the source `filters/` tree should never be touched, regardless of what root the function is called with.