Remove regexps in join_template plugin

[goshansmails]

Fixes #730

Benchmarks on same input:

goos: darwin
goarch: arm64

mixed res:
panic start with regex - 376828 ns/op
panic start without regex - 2397 ns/op
difference is 157 times

mixed res:
panic continue with regexp - 1257014 ns/op
panic continue without regexp - 16440 ns/op
difference is 76 times

negative res:
panic start with regex - 1441540 ns/op
panic start without regex - 6560 ns/op
difference is 219 times

negative res:
panic continue with regexp - 6450680 ns/op
panic continue without regexp - 46257 ns/op
difference is 139 times

[codecov]

Codecov Report

Attention: Patch coverage is 91.44737% with 13 lines in your changes missing coverage. Please review.

Project coverage is 73.70%. Comparing base (e02c4a6) to head (4ed0f46).
Report is 31 commits behind head on master.

Files with missing lines	Patch %	Lines
plugin/action/join_template/go_panic.go	90.37%	9 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #731      +/-   ##
==========================================
+ Coverage   72.28%   73.70%   +1.42%     
==========================================
  Files         137      145       +8     
  Lines       13985    15347    +1362     
==========================================
+ Hits        10109    11312    +1203     
- Misses       3311     3412     +101     
- Partials      565      623      +58     

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