add Grow function

[elee1766]

When using jx.GetEncoder() with streams, i constantly need to add this snippet

	enc := jx.GetEncoder()
	if cap(enc.Bytes()) < 4096 {
		enc.SetBytes(make([]byte, 0, 4096))
	}

i actually appreciate giving the user control over the allocation here - but i thought borrowing the Grow function from bytes.Buffer might be more useful. then i could do

	enc := jx.GetEncoder()
	enc.Grow(4096)

which would be more efficient, and also is much easier to read / understand.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (d2a6a27) 100.00% compared to head (043c94a) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #82   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           43        43           
  Lines         3187      3193    +6     
=========================================
+ Hits          3187      3193    +6     

Files Changed	Coverage Δ
enc.go	100.00% <100.00%> (ø)
w.go	100.00% <100.00%> (ø)

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[ernado]

LGTM, please update git commit messages