[chore][chloggen] Rename 'Context' to 'Config' and move to dedicated package

[djaglowski]

This PR follows #368 and #369 to further prepare the chloggen tool for the introduction of a repo-level configuration file.

• Rebrands the notion of a "Context" as a "Config", but stops short of actually introducing the config file.
• Creates a chloggen/internal/config package and moves former "Context" files into this package.
• Moves RepoRoot to cmd.repoRoot. The function is related to the "Context" of the tool, but as a "Config", it makes more sense that the value is provided.
• Further normalize tests across sub functions.

[codecov]

Codecov Report

Patch coverage: 83.33% and project coverage change: +0.25 🎉

Comparison is base (9228e92) 61.63% compared to head (1855b57) 61.88%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #370      +/-   ##
==========================================
+ Coverage   61.63%   61.88%   +0.25%     
==========================================
  Files          48       48              
  Lines        2330     2330              
==========================================
+ Hits         1436     1442       +6     
+ Misses        743      739       -4     
+ Partials      151      149       -2     

Impacted Files	Coverage Δ
chloggen/cmd/root.go	81.25% <60.00%> (-10.75%)	⬇️
chloggen/cmd/update.go	64.00% <83.33%> (+6.00%)	⬆️
chloggen/cmd/new.go	78.78% <100.00%> (ø)
chloggen/cmd/validate.go	84.21% <100.00%> (+15.78%)	⬆️
chloggen/internal/chlog/entry.go	76.92% <100.00%> (ø)
chloggen/internal/config/config.go	100.00% <100.00%> (ø)

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