parser: support cast as double

[wshwsh12]

What problem does this PR solve?

Related TiDB PR: pingcap/tidb#11443
Support cast(xxx as double).
Mysql support the parser in 8.0.17.
https://dev.mysql.com/doc/refman/8.0/en/cast-functions.html

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[codecov]

Codecov Report

Merging #399 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #399   +/-   ##
=======================================
  Coverage   70.22%   70.22%           
=======================================
  Files          32       32           
  Lines        7450     7450           
=======================================
  Hits         5232     5232           
  Misses       1703     1703           
  Partials      515      515

Impacted Files	Coverage Δ
parser.go	70.58% <ø> (ø)	⬆️
mysql/util.go	60% <ø> (ø)	⬆️
types/field_type.go	64.89% <100%> (+0.37%)	⬆️
ast/ddl.go	77.91% <0%> (-0.04%)	⬇️
misc.go	96.42% <0%> (ø)	⬆️

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[kennytm]

Please include a test case showing that CAST(x AS DOUBLE) is now a valid expression.

Rest LGTM.

CAST(x AS FLOAT), CAST(x AS FLOAT(p)) and CAST(x AS REAL) are also added in 8.0.17, would you mind at least supporting CAST(x AS FLOAT) in this or a future PR? Thanks.

[wshwsh12]

Please include a test case showing that CAST(x AS DOUBLE) is now a valid expression.

Rest LGTM.

CAST(x AS FLOAT), CAST(x AS FLOAT(p)) and CAST(x AS REAL) are also added in 8.0.17, would you mind at least supporting CAST(x AS FLOAT) in this or a future PR? Thanks.

UPDATE. PTAL @kennytm
And I will try to support CAST(x AS FLOAT) in next PR.

[kennytm]

LGTM

[zz-jason]

LGTM