Use different file extension to prevent conflicts

[swissspidy]

• No conflicts with wp-content/languages/{$locale}.php which WP supports for loading custom logic
• Avoids deleting wrong files in uninstall.php (i.e. index.php or wp-content/languages/{$locale}.php)

Tested using wp plugin uninstall performant-translations --skip-delete --deactivate

Fixes #77

[swissspidy]

Performance Test Results

Performance test results for 95160c8 are in 🛎️!

WordPress Admin

Locale	Scenario	wp-memory-usage	wp-total
en_US	Default	16.80 MB	147.08 ms
de_DE	Default	33.51 MB	198.75 ms
de_DE	Ginger MO (MO)	21.46 MB	175.55 ms
de_DE	Ginger MO (PHP)	18.43 MB	149.68 ms
de_DE	Ginger MO (JSON)	21.46 MB	171.24 ms

Twenty Twenty-Three

Locale	Scenario	wp-memory-usage	wp-total
en_US	Default	15.85 MB	137.00 ms
de_DE	Default	29.39 MB	184.50 ms
de_DE	Ginger MO (MO)	19.33 MB	165.72 ms
de_DE	Ginger MO (PHP)	17.02 MB	146.07 ms
de_DE	Ginger MO (JSON)	19.32 MB	159.75 ms

Twenty Twenty-One

Locale	Scenario	wp-memory-usage	wp-total
en_US	Default	15.38 MB	113.11 ms
de_DE	Default	28.98 MB	160.19 ms
de_DE	Ginger MO (MO)	18.87 MB	142.97 ms
de_DE	Ginger MO (PHP)	16.56 MB	118.95 ms
de_DE	Ginger MO (JSON)	18.87 MB	136.24 ms

[codecov]

Codecov Report

Merging #84 (a40986f) into main (5763cb7) will not change coverage.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               main      #84   +/-   ##
=========================================
  Coverage     92.21%   92.21%           
  Complexity      198      198           
=========================================
  Files             7        7           
  Lines           475      475           
=========================================
  Hits            438      438           
  Misses           37       37           

Flag	Coverage Δ
default	92.21% <100.00%> (ø)
multisite	25.47% <100.00%> (ø)

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

Files Changed	Coverage Δ
lib/class-performant-translations.php	95.77% <100.00%> (ø)

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