Provide pretty syntax for generic record typing

[tarao]

Before

import com.github.tarao.record4s.%
import com.github.tarao.record4s.typing.Record.Concat

def addEmail[R <: %, RR <: %](record: R, email: String)(using
  Concat.Aux[R, % { val email: String }, RR],
): RR = record + email

After

import com.github.tarao.record4s.%
import com.github.tarao.record4s.typing.syntax.{++, :=}

def addEmail[R <: %, RR <: %](record: R, email: String)(using
  RR := R ++ % { val email: String },
): RR = record + email

or

import com.github.tarao.record4s.%
import com.github.tarao.record4s.typing.syntax.{+, :=}

def addEmail[R <: %, RR <: %](record: R, email: String)(using
  RR := R + ("email", String),
): RR = record + email

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.76%. Comparing base (8c351c3) to head (a4f0a10).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #68   +/-   ##
=======================================
  Coverage   93.76%   93.76%           
=======================================
  Files          14       14           
  Lines         513      513           
  Branches      110      110           
=======================================
  Hits          481      481           
  Misses         32       32           

Flag	Coverage Δ
3	93.76% <100.00%> (ø)

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