Add functions in transform plugin to cover modify plugin

[timggggggg]

Description

Functions

Six new functions and one new option on capture:

function	signature
find_all	find_all(value, pattern, group: 0, limit: -1) — every match as an array
join	join(value, separator) — array of strings into one string
trim	trim(value, cutset) — strip characters from both ends
trim_left	trim_left(value, cutset) — strip from the start
trim_right	trim_right(value, cutset) — strip from the end
slice	slice(value, start, end: null) — substring by character, negative offsets
capture	new numeric_groups: false — also key groups by index: "0", "1", …

New files

• plugin/action/transform/stdlib/find_all.go + find_all_test.go
• plugin/action/transform/stdlib/join.go + join_test.go
• plugin/action/transform/stdlib/trim.go + trim_test.go
• plugin/action/transform/stdlib/slice.go + slice_test.go

Modified files

• plugin/action/transform/stdlib/capture.go — numeric_groups option
• plugin/action/transform/stdlib/registry.go — six registrations
• plugin/action/transform/stdlib/capture_test.go — numeric_groups tests
• plugin/action/transform/stdlib/resolve_test.go — callFn helper, binds arguments
  through the compiled signature so named defaults apply
• plugin/action/transform/transform.go — docs for the new functions
• plugin/action/transform/README.md — regenerated
• plugin/action/transform/doc_examples_test.go — compile-checks the new doc snippets

Fixes #1000