search: robust keyword scanning

Created by: rvantonder

This PR:

• fixes a bug where we don't always ensure spaces before/after a keyword that requires it (currently and and or)

I'm introducing a helper function to do lookahead and lookbehind for keywords. The lookbehind is avoidable, but a more involved change, so I will address this later (filed in #9812).

• the fix also makes keyword scanning more robust, so that queries like or or or also works
• fixes a bug where parsing would fail if the query only contained whitespace

Created by: codecov[bot]

Codecov Report

Merging #9811 into master will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #9811      +/-   ##
==========================================
+ Coverage   42.06%   42.07%   +0.01%     
==========================================
  Files        1340     1340              
  Lines       73727    73739      +12     
  Branches     6629     6629              
==========================================
+ Hits        31013    31026      +13     
+ Misses      39874    39872       -2     
- Partials     2840     2841       +1     

Flag	Coverage Δ
#unit	42.07% <100.00%> (+0.01%)

Impacted Files	Coverage Δ
internal/search/query/parser.go	94.83% <100.00%> (+0.37%)