Enhancement: Extract constant for indent characters

[localheinz]

This PR

• extracts an Indent::CHARACTERS constant that exposes a map of indent styles to the corresponding indent character

[codecov]

Codecov Report

Merging #384 into main will not change coverage.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##                main      #384   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
  Complexity       161       161           
===========================================
  Files             34        34           
  Lines            503       500    -3     
===========================================
- Hits             503       500    -3     

Impacted Files	Coverage Δ	Complexity Δ
src/Format/Indent.php	100.00% <100.00%> (ø)	9.00 <0.00> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update fa9eaae...1746a5c. Read the comment docs.