Add config extends and entry point exclusion

Profiles may inherit from another config file (extends: <path>): the
first profile of the referenced file is merged underneath (scalars from
the overlay win, slices concatenate base-first and de-duplicate, chains
resolve recursively, cycles are detected).

esbuild.excludeEntryPoints removes merged entry points by glob (** spans
path separators, * stays within a segment), applied after path
resolution so patterns and entries match in the same shape.

Lets product editions/flavors (e.g. the wmd4x Range Trader Pro edition)
build only their own entry set on top of a shared, merge-clean base
config. Includes unit tests and template docs.

templates/tpl.gowebbuild.yaml

@@ -1,6 +1,14 @@
+# A profile may inherit from another config file: the first profile of the
+# referenced file is merged underneath this one (slices concatenate base-first,
+# scalars from this profile win when set). Chains resolve recursively.
+# extends: ./.gowebbuild.base.yaml
 - esbuild:
     entryPoints:
         - frontend/the-app.js
+    # Entry points matching any of these patterns are removed after the
+    # merge. ** spans path separators, * stays within one segment.
+    # excludeEntryPoints:
+    #   - ./frontend/src/apps/experimental/**
     outdir: ./frontend-dist
     sourcemap: 1
     format: 3