Fix newline bug on folded scalar

[ultimateanu]

Fixes #313

Background

Issue #313 notes that an extra newline is being added to folded block scalars. This is because the yaml_emitter_write_folded_scalar function has this line:

    k := 0

This index variable is used when doing the look ahead from the current point in the scan to determine if we need to add a newline. I believe this is based on libyaml's emitter.c. However in that c library the checks using k are against a custom string type yaml_string_t which maintains an internal offset as evidenced by this line:

    while (string.pointer != string.end)

But in golang, by using k := 0 we are always checking the first character and not correctly doing a look ahead to see if the next line has more linebreaks or blank spaces.

Changes

• Updated the index k which is used for the look ahead to use i to check from the current point forward
• Added bounds checks when using k
• Campground cleanup based on LSP suggests such as removing unnecessary breaks, using modern range/max, remove unused function

Testing

• Added unit tests including the case from from issue Non-deterministic output for indented multiline strings #313
• Ensured all existing unit tests pass

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[ultimateanu]

I did some minor code clean up based on LSP suggestions on pkg/yaml/emitterc.go. I can break that out into a separate commit or PR. FYI, I didn't see other PRs with multiple commits so I didn't do that.