Add possibility to set up highlighten prefix and postfix for snippet

[lavrd]

This PR adds a possibility to change highlight prefix and postfix for snippets.

[lavrd]

Thank you for review! Did these fixes and reduced test.

[lavrd]

@fulmicoton hi, what the status with this feature?

[lavrd]

@PSeitz hey, can you help with review, please?

[codecov-commenter]

Codecov Report

Merging #1422 (80d764e) into main (0eafbaa) will increase coverage by 0.01%. The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##             main    #1422      +/-   ##
==========================================
+ Coverage   94.47%   94.48%   +0.01%     
==========================================
  Files         319      319              
  Lines       59404    59460      +56     
==========================================
+ Hits        56119    56178      +59     
+ Misses       3285     3282       -3     

Impacted Files	Coverage Δ
src/snippet/mod.rs	95.16% <100.00%> (+1.59%)	:arrow_up:

... and 6 files with indirect coverage changes

[PSeitz]

@lavrd Thanks!