Enable the multiline flag in line splitter regex by default

[camdencheek]

Description of Changes

Enabling this flag lets us use the line start and end anchors in our multiline regexes by prepending the patterns with (?m) by default. This limitation has made it so we can't parse logs correctly in the past (situations where the line start pattern also appears inside the log). Also includes a little bit of code hygiene

Please check that the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• Add a changelog entry (for non-trivial bug fixes / features)
• CI passes

[djaglowski]

Log Files	Logs / Second	CPU Avg (%)	CPU Avg Δ (%)	Memory Avg (MB)	Memory Avg Δ (MB)
1	1000	3.500133	+0.43112993	31.501347	-2.4058456
1	5000	7.982934	-0.36209488	39.73626	-5.580147
1	10000	13.603685	+0.15510559	54.469963	-0.15193939
1	50000	62.604374	-0.0519104	183.29015	-6.3965454
1	100000	124.04214	+3.7212296	292.41446	-0.7669678
10	100	5.0173883	-0.3448081	30.746632	-1.6877708
10	500	11.1726885	+0.10339737	40.647224	-1.6096458
10	1000	17.741724	+1.6552391	53.94922	-0.11530304
10	5000	69.2255	+1.5334549	179.89804	-0.52882385
10	10000	131.16866	-2.8150024	284.55994	-23.778015

[codecov]

Codecov Report

Merging #96 into master will decrease coverage by 0.11%.
The diff coverage is 63.16%.

@@            Coverage Diff             @@
##           master      #96      +/-   ##
==========================================
- Coverage   73.29%   73.18%   -0.11%     
==========================================
  Files          70       70              
  Lines        4245     4243       -2     
==========================================
- Hits         3111     3105       -6     
- Misses        863      868       +5     
+ Partials      271      270       -1     

Impacted Files	Coverage Δ
operator/builtin/input/file/file.go	77.99% <63.16%> (-0.76%)	⬇️
operator/builtin/input/file/read_to_end.go	59.70% <0.00%> (-2.99%)	⬇️

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