feat: add RESET_BACKGROUND and RESET_FOREGROUND

[innocenzi]

While working on Tempest's <style>, I realized that nested style tags wouldn't work properly because </style> would reset all styles.

When looking for a solution, I discovered that terminal codes 49m and 39m could reset only background and foreground colors, respectively.

This pull request adds those, since I need them in Tempest. :)

NOTE: I'm also renaming NON_REVERSE_TEXT to RESET_REVERSE_TEXT. While a breaking change, I checked and Tempest doesn't use it yet.

[coveralls]

Pull Request Test Coverage Report for Build 11827599007

Details

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 92.601%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/Themes/TerminalStyle.php	0	1	0.0%

Totals
Change from base Build 11817055171:	0.0%
Covered Lines:	1577
Relevant Lines:	1703

---

💛 - Coveralls

[brendt]

Thanks! Tagged: 2.11.2