`toolchainRequirements` recipe entry

DUB now supports a new entry in `dub.json` or `dub.sdl` to restrict the versions of
DUB and of compilers supported by a package.

dub.json:
---
{
    "toolchainRequirements": {
        "dub": "~>1.10",
        "frontend": ">=2.068 <2.083"
    }
}
---

dub.sdl:
---
toolchainRequirements dub="~>1.10" frontend=">=2.068|<2.083"
---

Supported entries in `toolchainRequirements` are:
$(UL
    $(LI dub)
    $(LI frontend)
    $(LI dmd)
    $(LI ldc)
    $(LI gdc)
)

Each can contain a
$(LINK2 https://dub.pm/package-format-sdl.html#version-specs version specification),
where DMD-like versions are supported in addition to SemVer versions. For
compilers, instead of a version specification, the keyword `no` can also be used
to indicate that the compiler should not be used for this package.

Example scenario:$(BR)
Package that needs DUB>=1.12, and that will only build with LDC>=1.10:

dub.json:
---
{
    "toolchainRequirements": {
        "dub": ">=1.12",
        "dmd": "no",
        "gdc": "no",
        "ldc": ">=1.10"
    }
}
---

dub.sdl:
---
toolchainRequirements dub=">=1.12" dmd="no" gdc="no" ldc=">=1.10"
---