Make MATH variable

[be5invis]

There are cases that we need to bolden the entire equation, like, equations in titles:

DISCLAIMER: This is NOT a proposal from Microsoft, just from me.

[stone-zeng]

See firamath/firamath#20.

[be5invis]

Some concept:

MATH Header, version 1.1

Type	Name	Description
uint16	majorVersion	Major version of the MATH table, = 1.
uint16	minorVersion	Minor version of the MATH table, = 1.
Offset16	mathConstantsOffset	Offset to MathConstants table - from the beginning of MATH table.
Offset16	mathGlyphInfoOffset	Offset to MathGlyphInfo table - from the beginning of MATH table.
Offset16	mathVariantsOffset	Offset to MathVariants table - from the beginning of MATH table.
Offset32	itemVariationStore	Offset to Item Variation Store

MathValueRecord

Type	Name	Description
int16	value	The X or Y value in design units
Offset16	deviceTableOffset	Offset to the Device or Variation Index Table – from the beginning of parent table. May be NULL.

DISCLAIMER: This is NOT a proposal from Microsoft, just from me.