Boost: Move the optimize part of LCP into a new class

[donnchawp]

I moved the optimize part of the LCP code into a new class to separate it from the base LCP class that sets things up. As suggested by @dilirity.

TODO: a factory method to generate an optimizer based on LCP data. Props @haqadn here.

// Create an optimizer from a factory function that returns new Background_Image_Optimizer( $lcp_data['element'] ) or new Img_Tag_Optimizer( $lcp_data['element'] )
$optimizer = $this->create_optimizer_from_data( $lcp_data );
$optimizer->optimize( $buffer );

Proposed changes:

• Create a new class called LCP_Optimize
• Move optimize, optimize_viewport and optimize_image_tag functions into the new class.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
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Does this pull request change what data or activity we track or use?

Testing instructions:

• Test LCP on your local test site.

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Code Coverage Summary

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/plugins/boost/app/modules/optimizations/lcp/class-lcp.php	0/69 (0.00%)	0.00%	-28 💚

1 file is newly checked for coverage.

File	Coverage
projects/plugins/boost/app/modules/optimizations/lcp/class-lcp-optimizer.php	0/29 (0.00%) 💔

Full summary · PHP report · JS report

_{If appropriate, add one of these labels to override the failing coverage check: Covered by non-unit tests Use to ignore the Code coverage requirement check when E2Es or other non-unit tests cover the code Coverage tests to be added later Use to ignore the Code coverage requirement check when tests will be added in a follow-up PR I don't care about code coverage for this PR Use this label to ignore the check for insufficient code coveage.}