feat: replace the template engine with an in-house parser

[favonia]

This implements #223.

[codecov]

Codecov Report

Merging #233 (175f358) into main (f216a7c) will increase coverage by 0.74%.
The diff coverage is 96.55%.

@@            Coverage Diff             @@
##             main     #233      +/-   ##
==========================================
+ Coverage   84.55%   85.30%   +0.74%     
==========================================
  Files          28       29       +1     
  Lines        1774     1932     +158     
==========================================
+ Hits         1500     1648     +148     
- Misses        266      273       +7     
- Partials        8       11       +3     

Flag	Coverage Δ
unittests	85.30% <96.55%> (+0.74%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
internal/api/ttl.go	100.00% <ø> (ø)
internal/domainexp/lexer.go	87.50% <87.50%> (ø)
internal/api/cloudflare.go	100.00% <100.00%> (ø)
internal/config/config.go	98.76% <100.00%> (-0.20%)	⬇️
internal/config/env.go	98.58% <100.00%> (+0.05%)	⬆️
internal/domain/domain.go	100.00% <100.00%> (ø)
internal/domainexp/parser.go	100.00% <100.00%> (ø)
internal/updater/updater.go	100.00% <100.00%> (ø)

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