Escpos

This change updates the supported PHP versions, and contains some breaking changes to improve compatibility with newer PHP installs.

Breaking changes to users who are upgrading:

- Support for PHP 7.0. 7.1 and 7.2 has been removed been removed. PHP 7.3 or higher is required from this version.
- Source files now use `declare(strict_types=1)`, which will cause errors for callers which are not using the correct scalar types.

Bug fixes:

- Using the GD library to load images would result in an error on PHP 8. This was due to a change to the GD interface from PHP 8.0.
- The example scripts now refer to `vendor/autoload.php`, so they will work if this repository is checked out and used standalone.
- PHPUnit has been updated to version 9.0.

Breaking changes for developers who are upgrading:

- Support for PHP 5.x and HHVM has been removed. PHP 7.0 or higher is required from this version.
- The `intl` PHP extension is now required ([see docs](https://www.php.net/manual/en/book.intl.php)). This replaces the use of `mbstring` and `iconv`, which are no longer used.
- [composer](https://getcomposer.org/) is now required to correctly load dependencies, where it was previously optional.
- `Printer::text()` will now throw an `Exception` if invalid UTF-8 is supplied. Use `Printer::textRaw()` if you are encoding text manually.
- Printer-specific sub-classes of `CapabilityProfile` have been removed (eg. `StarCapabilityProfile`). Use a named profile with `CapabilityProfile::load()` instead.
- The API now uses scalar and return type hinting in some places, which makes it more strict than it was previously.

New features:

- Inclusion of new printer profiles from [escpos-printer-db](https://github.com/receipt-print-hq/escpos-printer-db).
- [gfx-php](https://github.com/mike42/gfx-php) has been added as a dependency, which allows image processing to work without `gd` or `imagick` being present in most cases.
- A new `MultiplePrintConnector` has been added, which allows developers to route identical print data to multiple printers simultaneously.

Experimental features:

- A new `UnifontPrintBuffer`, which uses glyphs from a GNU Unifont hex file as an alternative to the on-board printer fonts. See the included `unifont-print-buffer.php` example for usage.

Bug fixes:

- Added checks where temporary files are created, to give clearer error messages on setups where the temp directory is misconfigured, see 362

New features:

- A RawBT connector and example

Newly tested printers:

- Bixolon SRP-350Plus
- CHD TH-305N
- Elgin i9
- Everycom EC-58
- Gainscha GP-2120TF
- SAM4S GIANT-100DB
- SEYPOS PRP-96
- Solux SX-TP-88300
- SNBC BTP-R880NPIII
- TVS RP45 Shoppe
- Zjiang ZJ-8250

New features:

- Upside-down printing is now implemented 615
- Include updates from [escpos-printer-db](https://github.com/receipt-print-hq/escpos-printer-db)

Bug and code fixes:

- Modify validation code in `WindowsPrintConnector` for PHP 7.3 compatibility
- Add to type hints where possible, replace old usages in examples, and correctly label the namespace for some exceptions

Newly tested printers:

- OKI RT322
- ithaca ITherm 280
- Rongta RP80USE
- Sewoo SLK-TS400
- Approx APPPOS80AM
- QPOS Q58M
- HOIN HOP-H58
- Xeumior SM-8330
- Birch PRP-085III
- 3nStar RPT-008
- POSLIGNE ODP200H-III-G

Bug and code fixes:

- `FilePrintConnector` throwing warnings if it was closed twice. 595
- Typo in `Printer.php`, fixed by wavoe

New printers tested:

- Bematech LR2000E
- DR-MP200
- Partner Tech RP320
- Xprinter XP-365B
- Zjiang ZJ-8220

This point release includes an updated copy of [receipt-print-hq/escpos-printer-db](https://github.com/receipt-print-hq/escpos-printer-db), so that character encoding improvements are accessible on these printers:

- TM-T88IV
- TM-T88IV-SA