chore(developer): kmcmplib and kmc-kmn/kmw have shared errors with different strings

[mcdurdin]

There are two shared errors between kmcmplib and kmw compiler which have different message strings. These should be consolidated, or split into separate errors:

  // kmcmplib: static Error_InvalidBegin                                  = () => m(this.ERROR_InvalidBegin, `Invalid 'begin' command`);
  static Error_InvalidBegin = () => m(this.ERROR_InvalidBegin,
    `A "begin unicode" statement is required to compile a KeymanWeb keyboard`);
  static ERROR_InvalidBegin                                   = SevError | 0x00B;

  // kmcmplib: static Warn_DontMixChiralAndNonChiralModifiers              = () => m(this.WARN_DontMixChiralAndNonChiralModifiers, `Don't mix the use of left/right modifiers with non-left/right modifiers in the same platform`);
  static Warn_DontMixChiralAndNonChiralModifiers = () => m(this.WARN_DontMixChiralAndNonChiralModifiers,
    `This keyboard contains Ctrl,Alt and LCtrl,LAlt,RCtrl,RAlt sets of modifiers. Use only one or the other set for web target.`);
  static WARN_DontMixChiralAndNonChiralModifiers              = SevWarn | 0x0A3;

Came out of #10867 work

Ideally, move the kmcmplib error messages themselves out of kmcmplib and into kmc-kmn.