Add the support for ARG in the native builder parser

[dkovba]

This PR adds the support for the ARG instruction in the native builder parser, implements the support for different kinds of ARGs, and performs the substitution of ARG variables in the supported instructions. Resolves #437.

Some features are currently blocked and not included into this PR:

• Native builder: add the current target platform we're building use it to set automatic platform ARGs #522
• Native builder: add the support for the BuildKit built-in ARGs #523
• Native builder: ensure pre-defined ARGs are excluded from the output of the history #524
• Native builder: add the support for stage references and ARG inheritance #525

[wlan0]

Only fix needed is to fix NSRange vs. String counting.

LGTM otherwise!

Also, the current impl doesn't support the following instructions right?

• ARG var= i.e. no value
• RUN $var i.e. without enclosing ${}
• ARG var2=${var1:-default}

Could we add an issue for these (if true)

[katiewasnothere]

Can we add guards for accessing the nodes in the stage?

[dkovba]

Crashing immediately with an index out of bounds error in a test provides a clear, specific failure message that pinpoints exactly what assumption was violated, whereas a guard might obscure the root cause. This will also make it easier to add new tests for added instructions in the future.