* `builder_has_action` renamed to `builder_start_action`
* `builder_report` renamed to `builder_finish_action`
* `builder_has_action` created to be a silent test for the action
* Added reasonably comprehensive documentation.
Fixes up scripts (except under /linux) to use `#!/usr/bin/env bash`
instead of `#!/bin/bash` or `#!/bin/sh` so that we don't end up with
the ancient version of bash supplied with macOS.
This became urgent with this PR, because of bash-4.xisms in
build-utils.sh, for example on line 572:
```
if [[ -v _builder_params[$e] ]]; then
```
Relates to #5816.
Moves folders under common/core/web to common/web:
* input-processor -> common/web/input-processor
* keyboard-processor -> common/web/keyboard-processor
* tools/recorder -> common/web/recorder
* tools/sentry-manager -> common/web/sentry-manager
* utils -> common/web/utils
Updates scripts and configuration to point to new folders.
