Make SVG scanning independent of PHPCS

[gudmdharalds]

Currently, SVG scanning functionality is implemented as part of PHPCS scanning, so that no SVG files are scanned if PHPCS is disabled. Also, results of SVG scanning is reported as PHPCS. This can be confusing to users and makes the statistics reported look a bit weird.

Further, auto-approvals of SVG files is performed when --svg-checks parameter is set to true, which is also used to indicate SVG scanning. This should be independent of each other: a) auto-approval of SVG files is done when configured to do so and no issues are found (using --auto-approve-svg-when-no-issues), and, b) issues in SVG files are reported if configured to do so (using --svg).

SVG scanning should be fully independent of PHPCS scanning, having its own statistics type and so forth. Auto-approval of SVG files should be independent of SVG scanning and reporting.

Relevant code:

• https://github.com/Automattic/vip-go-ci/blob/master/phpcs-scan.php#L411
• https://github.com/Automattic/vip-go-ci/blob/master/phpcs-scan.php#L479