feat: Add support for "UniGB-UCS2-H" encoding.

[kvii]

Fix #55

[ledongthuc]

Thanks for the PR — this is a clean fix for #55. The approach is correct: UniGB-UCS2-H uses big-endian UCS-2 source codes (the content-stream code is the Unicode code point, and the CMap maps it to a CID only for glyph selection), so decoding as UTF-16 is the right strategy. A few small cleanups before merge:

1. Reuse the existing utf16Decode helper instead of binary.Read

text.go already has the identical big-endian decode:

func utf16Decode(s string) string {
    var u []uint16
    for i := 0; i < len(s); i += 2 {
        u = append(u, uint16(s[i])<<8|uint16(s[i+1]))
    }
    return string(utf16.Decode(u))
}

Could you simplify ucs2Encoder.Decode to return utf16Decode(raw)? That removes the encoding/binary and unicode/utf16 imports from page.go. (Note: utf16Decode panics on odd-length input while your version truncates; content codes are always 2-byte aligned so it should be fine, but a length guard is welcome if you want to be defensive.)

2. Make the test test only the encoder

Decode receives the already-unescaped string — the lexer readLiteralString handles \( / \nnn. The bytes.ReplaceAll(data, []byte{0x5C}, []byte{}) conflates that with the encoder and would silently corrupt any legitimate code containing a 0x5C byte. Could you pass the decoded UCS-2 bytes directly (e.g. 0x00, 0x28 for ( and 0x00, 0x29 for )) and drop the bytes import? If you want to document the raw on-disk form, a comment is better than manipulating the data in the test.

3. Minor: copyright header

The new page_test.go uses Copyright 2014 The Go Authors, but the repo's other test files (read_test.go, lex_test.go, ps_test.go) have no such header, and this code isn't from the Go team. Mind removing it for consistency?

Everything else looks good — thanks again!

[kvii]

@ledongthuc You're right. I changed my code.

[ledongthuc]

@kvii Hi, it's good to merge now. Just got a conflict from page_test.go file
Can you solve it, or I can help to solve if you not sure.

[ledongthuc]

@kvii seems the fix is failed. Could you check again?

[ledongthuc]

looks good!