Allow deletion BOM inside plugin

[alexander-akait]

Sorry issue template not relevant.

For php we have PSR-1, PSR-2 and PSR-12 standards for code.
One of rule about BOM:

Files MUST use only UTF-8 without BOM for PHP code.

Unfortunately we don't have original text of file inside parse method https://github.com/prettier/plugin-php/blob/master/src/parser.js#L4.

Here prettier remove BOM

prettier/index.js

Line 77 in b449a52

const hasUnicodeBOM = text.charCodeAt(0) === UTF8BOM;

and add after reformatting

prettier/index.js

Line 125 in b449a52

if (hasUnicodeBOM) {

I do not know how to solve this problem better. Maybe:

1. Always pass original text to parser.
2. Add export option here https://github.com/prettier/plugin-php/blob/master/src/index.js#L55 (example - 'stripBOM').