Refactor term_program detection into public Terminal.detect_term_program(), adding tests

[jquast]

Follows up #420, to add detect_term_program() entrypoint.

This is needed for downstreams like gambaterm, they would call this "detect_term_program()" both for informational purposes, but to automatically apply the terminal-specific width corrections. In gambaterm's case, they make no use of alignment of emojis/CJK/complex languages, so it would be better not to call it.

[codspeed]

Merging this PR will not alter performance

✅ 31 untouched benchmarks

---

_{Comparing jq/more-xtgettcap-improvements (3a6ff69) with master (2a73ac0)}

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.74%. Comparing base (2a73ac0) to head (3a6ff69).

Files with missing lines	Patch %	Lines
blessed/terminal.py	91.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #421      +/-   ##
==========================================
- Coverage   97.75%   97.74%   -0.02%     
==========================================
  Files          13       13              
  Lines        4106     4117      +11     
  Branches      695      696       +1     
==========================================
+ Hits         4014     4024      +10     
- Misses         70       71       +1     
  Partials       22       22              

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.