feat(alphaTex): allow specifying tempo as a float in a string

[jonaro00]

Proposed changes

Stumbled upon a piece that has a BPM of 112.3(!), so I propose this change where the tempo number can also be a float in a string. (using additional dots and number breaks the parser in edge cases)

Checklist

• I consent that this change becomes part of alphaTab under it's current or any future open source license
• Changes are implemented
• Existing builds tests pass
• New tests were added

Further details

• This is a breaking change
• This change will require update of the documentation/website (or leave it as an easter egg :) )

[Danielku15]

I think it would be more natural for writers to write simply decimals \tempo 144.22 instead of strings.

You can follow the pattern with allow flags to change the parsing logic:

alphaTab/src/importer/AlphaTexImporter.ts

Lines 1928 to 1930 in 8593818

	this._allowTuning = false;
	this._sy = this.newSy();
	this._allowTuning = true;

Then in here you could use a new _allowFloats to read digits and points and then use parseFloat.

alphaTab/src/importer/AlphaTexImporter.ts

Lines 673 to 680 in 8593818

	private readNumber(): number {
	let str: string = '';
	do {
	str += String.fromCharCode(this._ch);
	this._ch = this.nextChar();
	} while (this.isDigit(this._ch));
	return parseInt(str);
	}

[jonaro00]

Sounds good! I'll get it done.

[jonaro00]

(we know that a dash is a digit if negatives are allowed)